It's always frustrating when you buy a piece of hardware (such as a computer) and then an upgrade/revision appears on the market a week later.
On the other hand, there are always new upgrades coming along; if you sit around waiting for the next upgrade, and the one after that, you may end up waiting forever. So the usual advice about this is good advice: buy a computer when you need one and when the currently available options are a good enough match for what you want. If you're satisfied with your purchase now, then that purchase doesn't retroactively become a bad idea when something better becomes available later.
However, hardware does tend to come in periodic cycles, and it can be helpful to get some idea of where in the current cycle you are. For example, if you know that upgrades to the standard-sized iPod have always been spaced 4 to 12 months apart, then you can be fairly confident that (a) if it's been a year since the last one, a new one is probably coming soon; and (b) if a new version has just come out, it'll probably be at least 4 months before the next version.
So the people at MacRumors have put together a Mac (and other Apple products) Buyer's Guide, showing the time between upgrades for all the current Apple hardware.
For example, that guide suggests that a new MacBook Pro is likely to appear relatively soon--it's been 242 days since the last significant upgrade, and the average time between upgrades over the past 5 years has been 186 days, with the maximum somewhere around 270 days. The MacBook Pro section of that page also lists recent rumors about forthcoming releases, most recently a rumor from a not-necessarily-reliable source suggesting that the next MBP upgrade will appear on February 12.
There's no guarantee, of course, that that page's predictions will be accurate. When it says "Don't Buy--Updates soon," it means that updates are expected soon based on average cycle length; but the MacRumors people don't have any certain knowledge of when or if a new version will appear. Could be days, could be months.
Still, anytime that you're contemplating buying new Apple hardware but you don't urgently need it, I recommend taking a look at that page to get a general sense of where in the release cycle the product you want is likely to be.