They've stated it will support Jabber. Depending on the Jabber server you connect to you can technically connect to yahoo, msn, aim, jabber, icq, all within that one Jabber connection using iChat (at least in theory because Jabber supports it). Not likely to support video or any of the other advanced features of each service though.
If that doesn't do it for you. Fire, Adium, Proteus all support multiple services. Fire and Adium are free, Proteus is shareware ($15 i believe). Do some searches and you'll find them on versiontracker.com