I don't think you'll see a G5 based eMac for quite some time. IBM/Apple are having enough trouble keeping up with PowerMac/iMac G5 orders. The eMac is Apple's extreme low end machine and currently doesn't even use the G4 to its full potential. I don't see any reason for Apple to make the eMac with a G5 in it. I don't think Apple could put a G5 in an eMac and still sell it for $799. Apple needs a $799 computer to try and compete with the $499+ Dells, HPs, Gateways, etc....
I think the next eMac update will be early next year. According to some vague rumors, its could be a significant update too, with a 1.5 GHz G4, larger capacity HD's, better graphic cards, etc.... Possibly even using a dual core G4 processor if available.