Yes, running AIM all day long will drain your battery. Anything that involves data will drain your battery almost more than anything else. I sometimes keep my MSN and Skype running in the background and its not too bad, but again, it wastes battery more than anything else.
To save battery life, makre sure you:
1. Turn your brighness down a little
2. have GPS off when not in use
3. have wifi off aswell when not in use
4. kill un needed running apps in the backgroung (use Advanced Task Manager, in market)
For steps 1-3, you can easily manage those with the widget "Power Control", it comes preinstalled in your Droid. Just long press anywhere on the screen, in the pop up select widgets, then select power control.
Some good IM apps are Fring and eBuddy.