Hi bennyboy,
I have three wrt54gs which I have been using very successfully with third party software loaded, when I purchased them I specifically looked for versions 1 to 4. This was at least 18 months ago so I am not up to date on version 6 but I have a note in my diary saying the v5 uses VxWorks OS and is not compatible with 3rd party firmware.
I purchased a v5 from Pcworld and was unable to load the firmware or something so took it back for a refund and found three version 1's on eBay for a good price. I had to ask the sellers for the serial numbers to determine the version but it was worth the effort.
I hope this helps but may not be totally accurate.
Cris