Well that was all a going a little too well. For some reason my Mac Mini desktop is stuck at ~4Mb/s download (with a decidedly asymmetric 15 Mb/s upload). Wireless signal is fine. Mac Mini next door runs full rate, MacBook right here runs full rate, WinXP runs full rate, Media Centre downstairs runs full rate.
Deleting entries, changing channels, PRAM reset, vigorous shaking and repositioning have no effect. Of course, which computer do I use the most...