Hey Dane,
Not to potentially hijack your thread, but I have had experiences where I'll deliberately turn the sync off because I did not want to sync a list, but everything gets pushed to SFDC anyway. It gets really frustrating when a bunch of duplicate leads get created that aren't attached to any particular object.
So to answer your question, at least anecdotally, yes, in my experience, Marketo does automatically create new leads in SFDC from lists uploaded into Marketo.
Austin