In 10.6.2 I can replicate the same bug described here ...
I have an imac & a macbook on my network. when I'm on my imac & i click on the sidebar to connect to my macbook, the same "connected as" name always comes up. i don't remember where this name came from (it's actually my mobileme email address) it's not a name i have listed under users who can share on my macbook. after connecting i have do disconnect then quickly click connect so i can enter in the username i want to use, if i don't then it automatically connects back under the name i don't want. i just want to reset the default "connect as" i guess I'm trying to say. i don't have this problem when connecting from my MacBookIt's a Snow Leopard bug. If you enable MobileMe Snow Leopard tries to authenticate with a remote machine using the MobileMe name instead of a username. I think it's some version of a directory service; in theory it probably lets you easily connect to a remote account with the same MobileMe credentials -- even if has a different username and password. It fails, however, if you try to connect to a share with a different associated MobileMe account.
When this bug hits you can't use the normal "Disconnect" or "Connect As" features. Editing the Keychain doesn't help. You can try clicking "Connect As" very quickly after Disconnecting and it might respond.
If you turn off MobileMe then you can connect and disconnect normally and the "Connect As" feature works. If you turn MobileMe back on the bug returns.
If you save files using this MobileMe authentication scheme on the remote share I think you may run into ugly permissions problems (such as 45,000 0 length files this evening on one failed write attempt).
Update: There's a Take Control book on permissions due out soon. The announcement blog post sounds ominous. I'm going to buy that book, I'll ask the author to check out this MobileMe bug too. Since Apple last updated their troubleshooting permissions document in 2008, and their screen shots go back to 10.2, the author has his hands full.
Update 3/11/2010: Given my recent experiences, I decided it was time to migrate Emiy's account to the new machine, and clean out the old accounts. I did a firewire migration and it went very quickly. After verification and testing I deleted her account on the old 10.5 iMac. On restart my old account on that machine had her user name, but all of my content. So the permissions were more than a bit screwed up over there. I deleted that account as well.