Well, this week has certainly been interesting with Pidgin development. On July 1, AOL made a small change to the OSCAR servers that affected our ICQ users. A lot of misinformation was spread in #pidgin about what really happened, so here's my best attempt to explain it.
Libpurple (and thus Pidgin and Finch) identified itself as ICQ basic 14.34.3000. Of course, we sent a version string that included our real identification ("Purple/2.4.2" for example). This has worked for us for years. We send the version bytes from official clients to the server because if we don't mimic an official client, we get locked out of some features. Which features, I'm not really sure, since I don't really know or care about the internals of the OSCAR protocol.
Suddenly we started seeing reports about ICQ no longer working in #pidgin. Upon investigation we discovered that apparently the client ID has been "retired", and any logins from that client ID are immediately met with a message from the server demanding an upgrade to ICQ6. The server then forcibly disconnects us. We catch this error and display a somewhat different message instructing our users to go to pidgin.im to get an update.
Stu Tomlinson found the version identification information for the most current official ICQ client and changed our identification. Magically logging in and holding a conversation works. This patch became the reason for the release of Pidgin 2.4.3.
So to summarize, this was NOT a protocol change, merely a tweak on the server side to disallow logins from "old" clients. I suspect this is to try to weed out older, buggier clients and get the users on current software.