you probably already know this but SNMP strings can be case sensitive...this has bitten me a few times in the past. like trying 'public' and it should be 'PUBLIC'.
have you tried using the same SNMP string coming in from the TRAP you're getting?
does Riverbed's SNMP config include a field like Windows, that is, like "ACCEPT SNMP PACKETS ONLY FROM ..." - maybe your Orion IP is incorrect in that area.
turns out I've been making an extremely bone-headed mistake.
The primary interface on the riverbed was down, and I'd failed to check it. D'oh!