To reiterate some of what has already been said:
For self-defense, a light is, at best, a tool that can momentarily blind.
That's it. Assuming it *is* bright enough to blind and not simply agitate, you get an opening of .5 to 5 seconds in which to follow up with a physical attack or get a head start running the hell away. As for which you do, I'll put it simply: unless you have a damn good reason for not doing so, run the hell away at your earliest opportunity. Not only is this safer, but in many cases it's the only legal option.
To give you an idea of the sort of mentality you need to have: I work security and when I train a newbie, one thing I always ask is: "If someone points a gun at your head, what do you do?" There's only one answer I'll accept: "Whatever he says." You need to remember that priority 1 is self-preservation, and that doesn't go hand-in-hand with getting one over on a bully (a common mentality.)
That BS aside, some lights can be used as a physical weapon as has been stated in the thread, but it's not something that comes naturally to most people. Look up some information on kubotans, koppo sticks, yawara, and batons (if you manage to find yourself carrying a 3D/4C or larger Maglite or similar light) to determine whether or not this is something you're willing to try. Make sure you gain an understanding of how this stuff really works, how you'd be able to use it, what effect it can have, what sort of trouble it can get you into, etc. before you commit to anything. And remember... it doesn't matter how many illustrations you read through, it's all meaningless without practice.
Now, while a flashlight can be useful given the right form factor and weight, it's often better to have something else on-hand that is capable of fulfilling the role of a physical defense tool. Personally, I carry a Sanford Sensa pen, the gel ink with stylus version (available at Office Max) which is a very capable kubotan but also functions exceptionally well as a pen. Any sufficiently sturdy pen will work (this one's made of stainless steel and weighs nearly as much as a minimag sans batteries...) There are other tools available, such as the Comtech Stinger, which are more effective (though also more obvious and legally questionable) than most improvised or dual-use implements, and easier to use, which you may want to consider.
A good resource for is
http://www.donrearic.com for general reading and ideas, as well as Google. In fact, there's a fair amount of info on the use of flashlights for self-defense. Read as much as you can; DR's word is not gospel, but he knows his poop.
Your first priority, however, will be to read and understand the ethics of weapon use. Some interesting reading:
http://www.selfdefenseforums.com/forums/showthread.php?t=9841
http://www.selfdefenseforums.com/forums/showthread.php?t=9671
http://www.selfdefenseforums.com/forums/showthread.php?s=&threadid=497&perpage=25&pagenumber=1
(I'm not a member, but I seem to find heaps of good info here.)
Also, some relevant writing on the subject of using a light to blind, and the use of a TID:
http://donrearic.com/tidinova.htm
I especially agree with the author on the GG&G's drawbacks. Note that a simpler, more traditional crennelated bezel will have less tearing and better force application. It might also have less legal murkiness attached (part of the light, or at least looks like it, versus nasty-looking aftermarket modification.)
Other commentary:
An OC spray is one of the most effective defensive tools possible for someone without hand-to-hand training. ASP makes a pen-style spray (called the Key Defender, I think) that can also be used as a kubotan, so if you want to go this route, that would be an excellent idea. FYI, I carry my pen in my back pocket and my shirt hangs over it... that'd also be a good way to carry this device. Bear in mind, however, that OC effectiveness varies with the individual, and some can even build up a resistance or even immunity to the chemical. Much like a bright light, it gives you an opportunity to get away - you do NOT use it offensively.
The new smaller TigerLight looks like an interesting tool. One of the options shown adds small protrusions around the front bezel that might make it an effective short club (for swinging) in addition to its spray/blinging capability.
Big Mag-Lites may be tempting, but the D cell models and Mag Charger are heavy enough that most people can easily screw themselves over by swinging and missing. C cell models are far better for anything shy of sheer impact force, but still difficult to wield. As with any large impact weapon, you need to consider weight. Also consider how you'd carry such a monstrosity. Same goes for the full-size TigerLight.
If you're really worried about a threat to your life, a small edged weapon might be worth considering. Most states do allow for concealed carry of small "common" pocket knives, and with practice/training they can be devastating in a fight. Even a keychain knife, properly applied, can save your life. You absolutely positively MUST have training, however.
To get back to the original subject... to use a light for blinding you can get away with as little as 40 lumens, but it depends on the amount of ambient light, proximity, and focus (ie. lux measurements have a big impact here, but with focus comes increased need for a good aim.) For instance, my McLuxIII-PD can momentarily blind under the right circumstances and it's probably no brighter than 55 lumens, possibly as low as 40, but then I'm not dealing with a lot of ambient light and I'm testing it on myself (arm's length or less.) In any case, you'll need to experiment in your own environment to determine how much is enough. I'd recommend picking up a Brinkmann Maxfire LX or a Dorcy Spyder. The Maxfire LX is available at Target and Wal-Mart stores for under $20 (and it can take Surefire lamp assemblies and some mods meant for the 6P Nitrolon, though possibly not the HOLA stuff) and the Spyder can be found for a comparable price (might be more expensive) at Lowe's and I think Costco. I think they're both over 60 lumens, the Spyder is the brighter of the two from what I've heard. There might be other good lights available locally (Target is now carrying Innovas in some stores.)
Good luck, and don't do anything stupid over something pointless.
