The Krill light, an electroluminescent "glow stick" powered by two AA batteries, might be a good option for when you're taking the dog for a walk. You haven't said how big or furry the dogs are, but for fairly big ones, you should be able to hang a krill from their collars without it getting in their way too much. It's very visible, especially if swinging around. Search on "krill" in this forum to find past comments by me and others.
For smaller dogs, you could perhaps attach a krill to the leash, so that it would bounce around under or behind the dog if it pulled loose from your grasp.
Most LED flashers seem to be fairly directional, so might not work too well if dangled from a collar.
For while the dogs are in the yard: reflective collars would be a good precaution to take. I can't think of any active lights that seem right for attaching to a dog 24/7.
I like 2d_edge's idea for a collar ringed
with LED's - another approach to that problem
would be electroluminescent wire, such as found at
www.coollight.com. EL wire is very cool stuff, though I don't know anything about its durability.
[editted to add the following]
This could be the ticket: an electroluminescent dog leash!
http://www.thatscoolwire.com/store/item.asp?Category=83&SubCategoryID=&GroupID=&SKU=FWSLL%2D316B