Thanks, this is helpful, but seems way more complex than what I should need in this situation. Defining a value on a standard picklist field in a standard object should be pretty straightforward.
It's like I asked you how to change the oil in my car and you said the only option is to rebuild the engine. There has to be a simpler way.
Dan Dlugos
I've also done this with a workflow rule in SFDC keying off a value in the subject line to update task type.
Although the field is standard in Salesforce, Marketo's support for tasks is limited to just a subset of standard fields. That's why you're getting suggestions with work-arounds of various complexity.
If you leverage tasks heavily you can also manage task creation entirely in SFDC (where you have access to the full range of fields) and trigger it off a data value change controlled by Marketo. The latter is an approach I often prefer when you want to add a lot of rich information to the tasks and populate several custom fields.