You don’t want to buy a product just before it is replaced by a faster version at the same price. That’s why MacRumor’s Buyer’s Guide is useful for Apple products. It currently shows that there are many iPods and computers that are due for revisions soon based on the time since the last revision. This coincides with recent rumors of a September 9th event to announce new products.
This does not predict exactly when a new version will arrive. Apple could come out with a small bump in the middle of a cycle. But if a revision is long overdue, you should consider waiting unless you need something immediately.