I had this same problem with another Linksys router using stock firmware and recently replaced it with a WRT54GL using the latest version of Tomato. The connection runs perfectly when the router is first turns on, but over the course of the day (or sometimes several days) the speed over wireless-G degrades dramatically (from 18+ Mbps when the router first starts to a few hundred Kbps later on).
This can't be related to anything other than the router because it happens on everything in the house that's wireless -- the Wii, the iPod Touch, the netbook, and Apple laptop, you name it. The connection slows to a crawl eventually, and one reboot later everything's running at top speed again.
Tomato is a stock installation. I had bumped the transmit power to 70 for a few days but dialed it back to the default 42 when this problem cropped up so I could eliminate that as a factor.
Is there something I'm missing? QOS settings or other Tomato configuration errors? Is there any long-term fix, or would I be better off investigating other options like powerline Ethernet (running cable isn't an option right now)?