If you get an iPad 2 and buy a bunch of apps, then next year you buy an iPad 3 and get rid of the old one, will you have access to your previously purchased apps?
As already mentioned they are tied to your account. I've gone through 4 different models of iPhone and 2 models of iPad and have always been able to transfer my apps.
Yes they are tied to your account, and under iOS5/iCloud, what ever iOS apps you buy your phone/computer/iPads will be pushed to all your devices simultaneously if you so choose.