When it goes to sleep, it shuts off all connectivity to the USB bus, hence your loss of wifi.
I have an ibook g4, 1ghz, running 10.5. I replaced the old airport card, and now use a usb "n" wifi dongle, and it's a lot faster. Problem is that every time the laptop goes to sleep, I have to manually restart the wifi. Easiest way I found is to just unplug it, and re plug it back in, but it's time consuming. Does anyone have any ideas how I can get it to automatically start after it wakes up? And thanks!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic