UPDATED 11:45 a.m., June 24, 2009: As I just found out via Twitter, what worked yesterday won't work today. If you've updated your client with the new version, but used the IP address below to fix Yahoo IM in the previous version, you must delete that IP from the Pager Server in Pidgin or the Login Server in Adium and restart the client.
Instant-messaging stalwarts Pidgin for Windows and Adium for Mac have updated to correct a Yahoo Instant Messenger connection error. Yahoo announced earlier this month that it was going to phase out support for Messenger versions 6.0 through 7.5 by August, and it seems that part of that process caused Pidgin and Adium to not connect successfully to Yahoo's servers.
A work-around involved changing your pager server in Pidgin by going to Accounts on the menubar, then selecting Yahoo, and then Edit Account. Under the Advanced tab, changing the Pager Server to 188.8.131.52 and restarting Pidgin would allow for a successful connection. In Adium, the fix was to change the Login Server to that IP, under Preferences, Accounts, and then the Yahoo IM Options panel.
The new versions of Pidgin and Adium will connect to Yahoo without manual adjustments. Pidgin Portable has also been updated to support the Yahoo fix, but Portable Adium had not yet been updated at the time of writing.