Hi,
I have some information from an iChat 6 Buddy of mine.
His action is not to Enter any Passwords when this Pops up but to use the Cancel button.
If he then clicks to log in again and it works without asking for the Passwords.
On another note he rarley resorts to deleting any .plist to fix any issue he may have.
Howveer he has noted that in Lion in his /Users/(Shortname)/Library/Preferences (Home Folder/Library/...) he has several tens of .plists for iChat. (Many repeating the names)
He has Property List Editor (Part of the additional apps the Developer Tools install adds to you computer) which should open the .plists.
At this point some are identified as Aliases by Property List Editor - but there is no marking on the icon that they are Aliases.
He also notes that the com.apple.ichat.Yahoo.plist seems to have been created even though he has not set up an Yahoo Account (And I can't remember if iChat 3 set up a Jabber one when that was added).
At this point I am not sure what this may tell us (Other than my Buddy uses his computers differently to me) but the info may spread some light on the subject.
10:00 PM Friday; August 26, 2011
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb( 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously