Matthew Jackson,
Mine is usually showing 54mbps, but I just checked and it's showing 36, so I opened internet connect and, sure enough, I only have 10 bars instead of the full 15. And all the while, my menu bar AirPort icon has continued to show the full four bars. That's why I always check internet connect rather than rely on the icon.
To see if you can get more info, try flipping it over and checking for updates, that seems to strengthen the reported info for mine.
edit-
iMac is two feet from the AirPort base station, sometimes during the day the signal just gets weaker, I believe it's the isp or DSL line, as a high percentage of the time I have the 54mbps.
Message was edited by: myhighway