I have the same problem of the computer going to sleep and not waking up on a Dell Dimension 3000 on dual boot with XP and Ubuntu 11.1. On rebooting, the computer fails to recognise the USB wireless modem Cisco AM10, while upon reinstalling through WUBI, the wireless connection is restored. While I found a solution for the the sleep problem from this forum(GNOME screensaver bug affecting INTEL 82865G chipset), I would like to understand what is going on with the USB wireless modem when Ubuntu crashes. Any help is appreciated.
Found the fix for the USB modem. Every time after booting, have to remove the USB Modem and reinsert it. Ubuntu then finds the wireless network!