Keep an eye on the end of software maintenance release, which appears to be July 28, 2009 for all versions of PIX OS. According to Cisco, this is the last date they will release any maintenance versions or bug fixes for PIX OS. Hopefully there aren't too many major deficiencies in some of the older code trains (6.3 has been around quite a while now), but some consideration is probably warranted on the decision to run code that will no longer be patched by the vendor.
This is basically a repeat of what Cisco did with their VPN concentrator last year.
Regards,
Chad
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"You need to be concerned about everything; from those boots, to that box, to the ferret on the wall." -DeCarli