Hi,
I guess Cisco are doing the same excercise with PIX software as they did with IOS:
"To make it easier for customers to find the right Cisco IOS Software release with the latest functionality and most recent fixes, Cisco is updating the Cisco IOS? Software Center and removing a significant number of obsolete releases. The Cisco Software Center updates began in April 2005 and will be ongoing."
However, if you go to the bug toolkit you can still find bug information on all current and past PIX releases.
And finally - if you really need an old PIX release you could always raise a TAC case and get them to send it to you (as I did only a few weeks ago.)
HTH
Andrew.