Should I buy a new Mac now?
One of the hardest questions to answer with computers is "When is the right time to buy?". That question is made even harder by Apple's fanatical secrecy and incredible control of their supply chains. Unlike other computer manufacturers, NO ONE* knows when (or if) Apple will release a new version of one of their products.
The only time I'm 100% sure that you should buy an Apple product is immediately after its been released. So, buying the new iPad right now (April 2012) is a pretty safe bet. You can be sure that it's not going to be superceded by Apple anytime soon.
But what do you do otherwise? Well, it's a bit of a black art. You could religiously devour the thousands of Apple-related blogs on the internet (as I do). Alternatively, if you've got better things to do with your life (I hope you do), there is an alternative. MacRumors.com maintain a page that tracks when Apple releases or upgrades their products. Using that and other information they rate each product on how likely it is to be upgraded soon.
You can find their site here. Green means it's safe to buy. Red means a new version is imminent. Keep in mind that they are just guessing like you and me, but they are basing their guesses on something substantive. Unfortunately that's as good as you will find.
So, the next time you're considering buying some Apple equipment, drop by that MacRumors site. You won't "know" when Apple is about to upgrade something, but you'll be basing your actions on better information than you previously had. :)
* At least no one that is going to share that information with us.