Salesforce.com activity - parent field is Subject

I needed to be able to restrict the criteria for a Salesforce.com workflow rule so that it was only triggered if an activity belonging to a particular custom object was changed.

To identify the object type I wanted to check the first 3 characters in the GUI field that's equivalent to the API WhatId.

Finding out from googleland which field I should use for this was surprisingly fruitless. Wrong choices:

• Opportunity/AccountId
• Contact/LeadId

Through trial and error it turns out that Subject was the right field! (!?!?Subject!?!?)

So my simple filter looks like "Subject starts with a99"