My SMC Barricade SMC2804WBR router (w/802.11b/g) goes to sleep if my machines don't talk to the outside world for awhile. All network connections have "limited connectivity," and it stops giving out IP addresses via DHCP. Power cycling the unit fixes the problem temporarily.
I called customer service a couple of months ago, and a nice Indian man sent me new (unreleased) firmware as a fix -- but the problem is still happening.
So finally, I set up one of my machines to ping echeng.com every 10 minutes, which in theory will keep the router alive. If that doesn't work, I may have to get a new one. Computers suck.