The basic rule of gadget-buying happiness is this: Don't buy anything after it's crossed the halfway point in its life cycle. That's the sucker line.
When it comes to iPods and iPhones, the lifecycle is easy to grok: Apple produces new models like clockwork. Every September, new iPods emerge, tweaked this way or that way. And every June, there's a new iPhone. Apple hasn't produced multiple generations of iPads for us to pinpoint precisely when the next version will emerge, but since it uses mobile guts like the iPhone - and those mobile guts advance technologically at the same speed - it's reasonable to assume the iPad will see yearly updates just like the iPhone. Also, the steadily rising volume of tidbits about the next iPad indicate it's coming soon, lining up with a yearly update cycle.
If you're going to buy an iPad. or any gadget with a yearly release cycle, the best time to buy them for the maximum gadget newness-happiness quotient is within six months of their release. After the six-month mark, only throw down your card in a time of need - you dropped your iPhone in a toilet or your iPad was used as an awkward frisbee by your three-year-old cousin. And if we're at the point we are now, a mere two months or so from a brand new model, do everything in your power to punt the purchase. Use a crappy flip phone. Borrow a friend's Galaxy Tab. Just wait.