Aha. Of course!
The netbios packets can't pass though NAT so they don't cross over my "internal" router.
I'm going to have a play tonight and swap some physical connections around.
The whole reason I've got two routers is because one is an ADSL modem router and the other is a cable router with wireless.
What I plan on doing is cable wireless router as a DHCP server but not have it actually doing any NATing. Hopefully then I can get them all on the same 192.168.0.x subnet and not have any problems. I assume the wireless router can route netbios packets from wireless to wired as they don't have to go through NAT.
I'm using simple Windows XP file sharing, btw.