We're going to cautiously hope that this is one of the many arbitrary limitations in Windows Phone 7
that Microsoft is planning to address in 2011 through updates
, but in the meantime, here's a little public service announcement: you can only receive push notifications for up to 15 apps at a time. The 16th app will receive an error internally, which it needs to handle gracefully and in turn communicate to the user in some way to let them know that the new installation won't be able to update live until you uninstall another push-enabled app on the device. Now, realistically, this will be a barrier that many users will probably never hit -- Clarity Consulting notes that there aren't even 15 push-enabled apps in the Marketplace yet -- so if Microsoft's really slick about this, it could have the situation resolved before it's even a real problem. Fingers crossed.