anyone remember how when the G5 was released, they continued to sell a legacy os9 bootable G4 for quite some time?
that said, with the iMac and powerbook/macbook pro, they continued to sell ppc versions alongside untill stock ran out, and at the very least, i think apple will plan it that way for the powermac/mac pro. whether or not they keep making them and selling them while waiting for more apps to switch is the question.
also keep in mind that if they release/announce these in august like we all think they will, that will mean they have completed the transition over a year ahead of scheduele; they can't expect all developers to get done that quickly.