Finding your WiFi/Airport connection broken after your machine wakes from sleep?
It appears that, with some non-Apple base stations where the Mac is using DHCP, it can forget its IP address during sleep, but does not request a new one. The best solution is to switch to using a fixed IP address, as even turning AirPort off then reconnecting to the network has little effect.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031007151244585