I had the same issue; 2 mo old 17" MBP. it seems common, and I spent a fair amount of time searching for a solution. I found many people with the same issue, and several different solutions, none of which worked for everyone.
I tried deleting the com.apple.internetconfigpriv.plist and com.apple.internetconfig.plist files; that made the problem worse. It would disconnect after a 5 minute pause in activity.
An upgrade to 10.6.1 also made it worse.
On the thought that it takes two to make a handshake, I reset my modem/router, a Westell 357, and restarted my Verizon DSL service from scratch. (WARNING: if you have Verizon, the Verizon installation package is incompatible with Firefox 3.5.3 and Safari 4.03. I found this out after I'd done the reset, and had to get a Windows laptop and use IE to do the setup.) I then went into the router properties and compared all the settings to the Airport settings, verifying that they matched.
Since that time, about 16 hours ago, I've had no issues. I've done the sleep/open routine a couple dozen times, with varying intervals from a few seconds to 7 hours.
I'm not sure why this worked; my guess is that it is related to the dynamic assigning of IP addresses by the router. The google search result that I find closest and most applicable to what I did is
this one. So if what I did doesn't work for you, try that. There is a list of things to try at the end of that article; resetting the modem/router is on that list.
I hope you get it to work, my friend.