I've read that this has been a problem for a few with the new Macbooks/ Macbook Pros and the Airport disconnecting after a few minutes of non-use.
I had this problem using a Linksys router and researched the various Apple forums and did not find a consensus to the cause of the issue.
In my situation, I downloaded Kismac and noticed that my router was still on the default channel which was full of signals from routers owned by neighbors. I switched channels and have had fewer issues, though I still occasionally have to run through the network assitant in the preferences tab as the Macbook sometimes decides to assign itself an IP address.
That said, it really could be a variety of things, from interference (I've heard even Bluetooth) to the specific router to problems with the Airport hardware itself.