Well, I don't think it's an App limit - I just had a bit over 30 Apps installed at once, and while I wouldn't recommend it for stability, it never errored during any install.
What's odd is that's the same error anyone would get if they tried to install an App still in Development - I'm not sure what the process is behind-the-scenes that prevents a non-friend from installing an App in Dev, then goes to allowing them to install once it's Live. If the check happens at install, on a per-person/per-install basis, then there might be a hitch there. Unfortunately, only a couple of people have reported this here - but then again, only developers would and most devs have modified their profiles to fit the requirements of the MDP beta phase (setting international settings to US, not blocking Applications from viewing their data, etc.).
I dunno, like Eddie I'm just shooting at stars. But any information you can provide would be helpful.