Just wanted to throw my experience out there that while upgrading 4 FWSMs from 2.x releases to 3.x releases I encountered more that a few issues when I loaded up the 2.1(4) MP software. The biggest issue was that if I rebooted the FWSM and not the 6500, at times I could not session into the FWSM. If I rebooted the entire switch, the FWSM was available again. Switches were running IOS with either a 12.2(33)SXH release or a 12.2(18)SXF release.
Again, just my experience in case it ever helps anyone.