Well, I think I managed to get this working. I believe the problem was ICS. I found this thread, and a CAB to install ICS here, in Post number 2:

Internet Sharing over WiFi - WMWifiRouter v0.91 (Jan 14, 200 - xda-developers

I'd suggest anyone else having problems with this use this CAB file to install ICS, as that seems to have done the trick for me.