A very good website that provides history and analysis on Apple's release cycles for its products such as Macbook, Macbook Pro, Macbook Air, iMac, Mac Pro, Mac Mini, iPod shuffle, iPod nano, iPod classic, iPod touch, iPhone etc

You wouldn't want to buy a product when the line-up is going to be refreshed in another week. So go to MacRumors: Buyer's Guide to check out!

If you happened to have bought Apple's old line-up product, Apple has a price protection for 14 days after the purchase. So you can bring it back to Apple Store for upgrade or change for pretty much free (unless the new line-up is more expensive that the one that you have bought). Terms and conditions might vary with different countries.

MacRumors: Buyer's Guide