this happened to me when I tried to enable push notifications for govoice. I think I had reached my limit at that point. I tried to disable notifications on other apps but it still didn't work. I also tried to uninstall and reinstall govoice, to no avail. I was scheduled to reset my phone anyways in order to install an sd card and I used that as a fresh start for notifications. Haven't hit the limit yet.
Maybe you can turn off notifications for some of your other apps and do a soft reset.
In my experiemce, WP7 just isn't dependable when it comes to push notifications (i've had to mostly give up on alphajax, spell it and weather channel because of this).
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