Why is it improbable you ask? The fact that all software that currently runs on Macs using the PowerPC chip architecture would need to be recompiled to run on Intel chips. No existing software would be able to run on the Intel Mac, and that isn't a good thing. Using WWDC to announce the move a year or so in advance to developers would give them plenty of time to get their software up and running on the new Intel Macs, but John points out who would buy a PowerPC Mac during that year? I know I wouldn't.
Finally, John points out that Apple makes more than Macs now adays and Intel makes many chips other than those processors that we see our Windows brethren using. Apple could very well be considering using Intel chips (heck, they already use Intel chips) but most likely not for Macs.