Avoid older LinkSys WRT54G, but I did get it working.
I was puzzled as to why my WRT54G (Vers. 6, Firmware v1.02.5) would not work with my MacBook Pro with wireless security turned on, when it worked fine with a WAP54G (wireless access point) I have on a different network. I read everything I could find on LinkSys and Apple finding only a few clues. Interestingly the LinkSys support solution does not work at all! The only reason I'm mentioning the WAP54G at all, is to show that the probem is not on my MacBook Pro, and lies within the WRT54G.
Trying all the clues I could find, and trying variations of those clues, I accidentally stumbled onto something that works. I was using variations of the LinkSys suggested solution where you setup the WRT54G wireless security to "WPA Personal", "TKIP", a shared key, and 1500. (I don't think the 1500 matters, but that was what was in their example, and I was willing to try anything at that point)
What I apparently accidentally found out is the WPA Shared Key needs to be 12 characters; this menu will let you enter more or fewer characters. A few times I just used a different key, and one time the MacBook suddenly took the key and connected! I backed up to see what was different this time, and the old key I was using was 11 characters while the new one had 12. I had been at this for some time, so I didn't expermient any more just being gratefull that it now works. Nowhere did I read about this limitation, on Apple or LinkSys web sites. What I've described here was the ONLY thing that worked on the WRT54G.
BTW, all the other modes like WEP don't work connecting a MacBook with the WRT54G. On the other hand, I can use any securtiy mode I want when connecting to my network that uses a WAP54G. This underlines that the problem is in the WRT54G.
A sidebar footnote: If you are thinking of running out and buying a WAP54G, be cautioned that product only work as an access point as a SERVER. It will not work as a CLIENT to access any wireless routers. (this is sort of in the "fine print" in the support area for this product) It does not issue DHCP addresses, and even if you hard code the IP settings it still won't work. The only thing way the WAP54G will work as a client is talking to another WAP54G that is the sever.