My wife is in graduate school, and the school ordered a large number of Macbooks for their students. All of these Macbooks shipped in the fall before Leopard was released. I upgraded my wife's laptop to Leopard, and it is fully updated to the most recent versions right now. However, all of the other Macbooks that the students received (every single one) are having the intermittent wireless connection problems. All of them are on Tiger except my wife's laptop, and all of the computers are having problems regardless of the type of wireless network they are joining (we have Airport Extreme at our house, and the problem exists just like in all other locations).
In January, I called Apple to give my info, but given the fact that I knew it was a widespread problem with the Macbooks, I didn't expect a fix immediately. The tech researched the issue a bit and said that they had received a large number of cases on this issue and assured me that they were working on it as a top priority. He took down all the info from my wife's computer to help with their troubleshooting. However, still no fix, which is leading me to believe that it is a hardware incompatibility issue that will take a recall of some sort. Generally, I've found Apple to be pretty quick (at least within a month or two at the longest) with their software fixes.
Hopefully, they come up with a fix soon, as all of these laptops have been a real pain for these students, all of whom spend most of their day researching documents online... However, I don't think there's much that the genius bar at an Apple Store or the tech support guys over the phone can do about this problem. Given the fact that it's happening on every single one of the Macbooks the school ordered (both on Tiger & Leopard systems), I'm not sure trading it in for another unit will be a fix for the problem.