One may have to physically look though---I have seen models like the 2600 that do not show up on Cisco.com, and one would have to physically look at the bottom, as the chassis serial number is the actual serial number. This holds true for the VPN Concentrators. Most switches will be in the sh version. The PIX/ASA will be a number in the sh version output, but you must add two more of what the first digit is. For example, if the PIX is "87746356", then the actual serial number (as it is on Cisco.com) will be "8887746356". That is only for the PIX/ASA.
Burt