I have found the same problem:
. 585v6 router
. Internet and all Internet-side diagnostics work ok
. LAN-side clients all DHCP correctly
. Game & application sharing works ok - Internet connections are NAT'd correctly through to a statically addressed LAN-side client.
. BUT....LAN-side clients cannot access Internet :-(
It's as though it needs a route added to pass traffic from 192.168.1.x to the Internet. Any ideas?