ceAssetSerialNumber (.1.3.6.1.4.1.9.9.92.1.1.1.2)
in the CISCO-ENTITY-ASSET-MIB
Probably best to walk it's parent table ceAssetTable (.1.3.6.1.4.1.9.9.92.1.1) to see what you want. For the class of hardware you're asking about there will be a lot of info in there.
The new(ish) SNMP Object Naviagator on CCO works well http://tools.cisco.com/Support/SNMP
