I'd say that the Rev B iMac G5s will be announced after the next revision of the PowerMacs. And that will be somewhere between now and the end of June.
Sometimes it's worth waiting for an update and sometimes one should just go and buy a machine.
I'm pretty sure Apple will wait for the euphoric honeymoon of the Mac Mini to die down and stock movements assume a normal pattern (say four months) before the revB iMac comes out.
It doesn't seem terribly likely that it would be a major revision, given that it's a new platform and the Powermac ceiling isn't all that high and isn't likely to be all that much higher.