If you want to use SNMP polling in MOM 2005, you have to use WMI with scripting to make that happen.  I don’t know if you have tried to use WMI to poll SNMP devices, but it is not very easy if you have to import a MIB.   I found an easier way to poll SNMP devices in MOM 2005 by using a freeware DLL and polling the SNMP devices via scripting.  The DLL is called System Scripting Runtime and it can do a number of things but most importantly it can do SNMP polling based off the OID on SNMP devices. 

In the Downloads section of the site is a file called SNMPPoll2005.zip that contains a sample script to poll a single IOD.  This script will log an event if polling the OID fails or if a logic condition is not met. The script is designed to run on an interval and another rule checks for a certain event ID and alerts on that. 

Argument 1 - DeviceName :target device (ex: 192.168.0.43)
Argument 2 - CommunityString : Community string (ex: public)
Argument 3 - OID :  The identifier to poll on the SNMP device (ex: 1.3.6.1.4.1.9.9.27)

System Scripting Runtime

Info: http://www.netal.com/ssr.htm

Download: http://www.netal.com/Software/SSR15.ZIP

SNMP Polling Script

Download: http://systemcenterforum.org/wp-content/uploads/SNMPPoll2005.zip