Apple tends to refresh it’s products every once in a while. Actually, most of the time these ‘(hardware) product updates’ normally have rather predictable product cycles.
With this knowledge in mind, one can estimate when it’s a good time to buy specific Apple products, and when it’s better to wait.
MacRumors’s Buying Guide will tell you :
Every current Apple (hardware) product is listed [ at the moment of writing this, they are : iPod Shuffle, iPod Nano, iPod Classic, iPod Touch, iPhone, iPad, MacBook Air, MacBook, MacBook Pro, Mac mini, iMac, Mac Pro, Cinema Display ], and every product is accompanied by one of these advices :
Buy now! Product just updated
Buy only if you need it – Approaching the end of a cycle
Don’t buy – updates soon