I'm not sure if this is a genuine bug or not but thought that I would log it in case others have similar issues.
New firmware installed approx a week ago on ASUS WL-500gp and was working fine for several days.
Users complained of not gaining access to Internet and upon investigating I was able to remotely log into the router (hence Internet was functional) but up time indicated that router had rebooted (or been rebooted by user) earlier that day. Since the reboot no users where showing in the "Connected Users" section.
Another reboot initiated by myself with no changes to the config and all came good.