Waiting ALWAYS helps with computers: they only get better and cheaper!
But you give up the use of the computer in the meantime. So the buyer has to make a judgment: how much do I want/need it now?
At some point you have to go for it, because there will ALWAYS be a better model on the way.
That said, there are probably TWO revisions to watch for:
1. A revised/faster PowerPC Mac Mini coming this summer (educated guesswork). Maybe soon. Maybe August. Hard to say--but I suggest waiting a little longer to see what news may come. It's very likely that the current model is not the last one before the Intel change.
2. A very nice Intel Mac Mini will arrive next year. Soonest: January. Latest: June. Maybe too long to be worth waiting for?
The main reason to wait isn't that the PowerPC will be obsolete. It won't for a LONG time. But rather, simply the fact that computers get faster and faster--and the Intel jump may be a speed boost in particular.