According to Jajcus and Ralph Meijer Google Talk has opened it's gates for other Jabber servers to federate with, a quick test showed it's really true:
[19:30] email@example.com accepted your presence subscription request
[19:30] firstname.lastname@example.org/Talk.v82751FB013 () is online:
WOW, at last. Now if only Google Talk could make use of all the transports out there...
UPDATE: You can find an official confirmation here.