Do you have any other logins to Yahoo with the same screen names ?
Another Yahoo capable app, another computer or a web based Login ?
There seems to be some evidence that one log in will log out a previous one to some degree. In other cases it seems to prevent login.
Try going to the iChat Menu > Preferences > Accounts and deselecting the Use this account tick box.
Then re-tick it (I.e log iChat out of Yahoo then try logging in again. Do this with no other Yahoo capable app open)
If you have had a web based log in did you log out before quitting the page ?
An AIM or Google Login have default Login periods of up to two weeks if you just leave the page without Logging out.
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