I am having trouble getting MarketPress working on my multisite with domain mapping. The ajax shopping cart seems to break when using domain mapping. Just get a "roundy roundy" when clicking on add to cart. Looking at one of the other discussions I set up a vhost to see if that sorted things out. No change.
I found a work around here.
I agree with Saunt Valerian that this is a significant bug and needs to get worked out.