Because Apple uses WiFi chips that are also widely used elsewhere, it is likely that the chips themselves work fine. However, any hardware must be controlled by a lot of software, and in this case the Mac OS X power management seems to be at the root of lost connectivity. If you are running Mac OS X Lion and are having this problem, this may help.
Users in the Apple forums have apparently been asked by router providers to disable the power management in order to not lose connectivity. Basically, it seems that there’s an issue between some routers and some Mac OS X Lion when the WiFi chip goes to sleep. When it awakes, the communication with the router doesn’t restart properly. Right now, it’s not clear who should fix what, but only OS X Lion users seem to complain.
A temporary fix used by some Mac users is to “ping” the router on regular interval to maintain network activity, and therefore avoid falling into “sleep” mode. This is a bit convoluted, but at the moment, nothing else seems to work. Have you had this issue?