Yahoo! has made a change on their server that prevents Adium 1.3.4 and the 1.4 betas from connecting. We have released a new beta, beta 7, which contains the fix.
As we’ve noted before, 1.4 will require Leopard. Those of you who are still hanging on to Tiger are currently stuck, as we do not yet have a fixed version for you. We’ll release a 1.3.5 shortly after Libpurple 2.5.7 comes out, as that version will also contain the fix. (Libpurple, for those of you who don’t know, is the library Adium uses to connect to Yahoo! and most other services.) The Adium 1.4 betas use pre-release versions of Libpurple 2.6.