Here's another cool tip sent to me by Milan Jajal, a support engineer in our Manageability group. If you're trying to create a new Event View using a custom rule and it's not showing up then this may be your issue:
Issue: When using the System Center Operations Manager 2007 Console to create a new Event View based on criteria of "generated by specific rules," you may not see some of the rules that you created manually.
Cause: This can occur if the rule is not configured with the category of EventCollection.
Resolution: Modify the Rule by going to the Rule properties and on the General tab, under Rule Category, select EventCollection as an option. Once this is done save the rule by clicking on OK. Now when you try to find this rule while creating a new Event View based on criteria of "generated by specific rules" it will show your rule.
J.C. Hornbeck | Manageability Knowledge Engineer
This is true for performance rules, and alert rules.... these should be set from the default, which is "Custom".