I'm also curious about this. First off, what is a Milestone? I'd like to have something that differentiates between a web visit and form fill. Is a form fill a milestone? I'd call it a Form Fill, or something like that. Maybe same category as an Event registration, which i wouldn't call a milestone or a form fill. To me a milestone sounds more like they've moved from a lead to an opportunity, or maybe they've had their first child
Update: I just typed in whatever i wanted as the Interesting Moment Type, and that value was saved in Marketo, and was also passed into our CRM, Zoho (i know, i know). However, it doesn't save as a new Type for future flow steps. That would be a nice feature.