Erica Harris had exactly the same story as you can read here: SFDC Professional Edition customers - sync change alert - please read!
It turned out that there is still what SFDC calls a one-way API is still available for a fee on the PE and is enough for Marketo to work. It seems that what they have put an end to is the availabilty of this API for free.
Anyway, thousands of third party products integrate with Marketo using the SOAP API and I strongly doubt that SFDC will force all the PE users to double their license costs...
Thanks. I will pursue my question on thread instead. Looks like the conversation took place on Jan 6, well before the Jan 31st cut off. Would be interested to see what happened with everyone else.