Hi Jonathan,
Your PIX is only vulnerable if you are running 6.3 code or earlier and have IPsec enabled (which is not enabled by default). The vulnerability is due to CSCef57566. Also, I would recommend taking a look at the follow link, which has all the details of this vulnerability:
http://www.cisco.com/warp/public/707/cisco-sa-20050412-icmp.shtml
As a workaround, you can disable IPSec, though this is probably not a viable solution for you. Therefore, to be protected against this vulnerability, you should upgrade to the latest 6.3.5.x interim release, or move to the 7.x or 8.x trains where this vulnerability does not exist.
Hope that helps.
-Mike