This seems kind of weird, and I haven't tracked it down enough to create a ticket yet, but I figured I'd throw it out there....
I have an AIM account that I've been using with Adium forever. I recently created a Bonjour account. Now the AIM screen name sometimes becomes the user name I chose for the Bonjour account, and the Bonjour name sometimes becomes my AIM screen name. This happens after a relaunch of the app. When the former happens, I obviously can't authenticate on AIM because it's trying to do it with with the wrong SN.
For the sake of troubleshooting, I created a new Adium user (hold option while launching Adium), created an AIM account, then created a new Bonjour account. The behavior from my normal user did not occur.
I created all new prefs when I started using the .8SVNs, so it's not a prefs importing issue from .7x.
A related issue is that when you create an account, Adium places a #.plist file in the ~/L/AS/Adium 2.0/Users/myUser/Accounts/ folder, which is fine, but, when you do delete an account, that #.plist does not get deleted. Since I had created and deleted several test accounts in the past on my default user, several #.plists had accumulated, and before I created the Bonjour account, I deleted the ones for accounts I had deleted. Might this be why it's getting confused?