Hi,
I can tell you that with my tests, I figured out that it's happening with all sorts of clients (MacOSX, XP, Vista). It appears only to be happing with the WAP54G (and now the WRT54GL you say). When I replaced the WAP54G with a WAP200, the errors disappeared with the same clients. I tested this on many locations with many different clients and everywhere the same results. It must be the WAP54G then.
I'm still using those AP's and I keep getting the error in the logs. It's indeed quite random. The error seems not harmful (although the sourcecode of freeRadius says the AP is broken). I haven't been able to link complaints of customers to this specific error, so I guess I should just stop paying attention.

I ruled out the possibility that overlapping channels are causing the problem. I tested it with overlapping channels and without. Nothing changes regarding the error.

I mailed Linksys about the problem, and they sent me a newer beta-firmware which isn't on their website. It's for the WAP54G ofcourse. If you want it, I put it on our website here: http://www.orxnet.org/files/WAP54G-Cisco-EU-EN-3.08.02.zip
It works, but doesn't give me the results I hoped for.

So, I don't have the solution, but I guess it's not really that bad. If you ever solve the problem I'd like to hear it! :)
Good luck!

Gr, Jelle