I carry a Mini AA with a 14500 in it on my keys, and a LED 2AA MiniMag (with the rebel) in my backpack, along with 2 L91 primaries (in case it's cold) and 2 Eneloops in a dealextreme battery holder. Takes up virtually no space (the maglite fits in a pen holder pocket in the backpack) and is extremely light. As an added bonus, I just ordered a Eneloop 2AA mobile booster/charger, so I'll always have a power source for my cell phone.
The batteries are interchangable with both lights, and between them and the backup batteries I should have enough light for weeks on the low modes.