More evidence that this is an issue on the Marketo side: From what I've seen, we have old lead records that for some reason were being sent from Marketo to SFDC as new lead records, and 20+ new records were being created by the Marketo user in each case, resulting in hundreds of new / dupe records total.
Of course, this triggered a number of workflow rules in SFDC, which sent out emails to our resellers and end users alike, which has angered some of our customers and partners.
Not good, and I'm still waiting for details on cause / resolution on why Marketo caused the Marketo user to behave erradically like this - or evidence that this was a breakdown on the SFDC side, which I'm just not buying.