Hello Scott,
I'm glad it helped to find a reason why it does not work. I've been successfull with my problem as well, Marketo by default trusts only peers with certificate authorized by some big CA as Symantec, Comodo etc. our certificate was signed only by our company root certificate.
Even though that my question about this was not answered here I've turned on support and they said that they will install our root certificate on our machine, so now I'm waiting for that. I hope that this will help to anyone who'll get to the same trouble as I.
Thanks to you all,
V.