After about 1/2hr of trying to figure out why the proc and exec directives don't work in Ubuntu's snmpd server (at least in Ubuntu 8.10), I finally found a reference to having to recompile and use a non-standard package because only a subset of all the service modules are compiled in the default package.
For reference, you can find out what the compile-time options are using
net-snmp-config --configure-options, and here are the default compile options in Ubuntu 8.10:
--with-mib-modules=host smux ucd-snmp/dlmod ucd-snmp/diskio ucd-snmp/lmSensors
The module I need (in order to monitor running processes) is ucd_snmp, which provides both the proc and exec directives. There's also the extend directive, from the NET-SNMP-EXTEND-MIB, but I can't even see what module that's added from right now.