I encountered this issue on my Windows 7 desktop (Windows 7 Home Premium - Service Pack 1). After reboot all of a sudden the default gateway of my wireless adapter was being wiped, and would show up completely blank (not as some have reported as 0.0.0.0, but actually blank) when inspecting the IPv4 properties of my static IP configuration.
I found the following link to a discussion about the issue:
netsh int ip reset
Rebooting after executing this command seems to have resolved the issue for me. It has been a few days and multiple restarts later, and my default gateway has remained intact. Some further details about my wireless adapter as well; it is a Linksys AE1000 USB adapter which, this issue excluded (which may or may not have anything to do with the adapter), has been rock solid for me for over 3 years of use. I have not experienced a flaky connection or fluctuating speeds as many reviews state.
Further information about the NetShell utility (netsh) and resetting TCP/IP settings can be found at the following Microsoft Knowledge Base article: