I appear to have just confirmed I have a kernel memory leak on this machine.
Before reboot: 496M of physical RAM out of 512M in use for processes, 300M of swap out of 512M used, total of around 850M of RAM usage and only 50M of it buffers and cache. After reboot: 295M of physical RAM used for processes, 210M used for buffers and cache, zero swap used. Same processes running, same number of browser windows open to the same pages, same four processes are the top memory users, they're still the only ones using over 1M of RAM, and they're using about the same amount as they were before rebooting.
So somewhere, the kernel had leaked half a gig of RAM....