I'm part of Marketo's Privacy Team. We handle email deliverability & compliance. One of the things we do is figure out issues like this. We have been doing some testing on this. We know that some opens are coming through just fine, but we don't yet have all the details to give you a definitive answer.
I would be interested to see what you're seeing in your own campagin statistics. Do you see a drop in opens? If you'd like to share confidential details, examples, or campaigns we should look at in our testing, drop me a line. I'm firstname.lastname@example.org.