There seems to seems to be a fault with the windows phone platform affecting a wide group of it’s users wherein in the last couple of days (most users report on 31st July) their calendars are not syncing between windows live and WP7.
The primary thread on the microsoft site seems to be this one:
If you’re being impacted, hit that page an press the “me too” button on the top right.
That thread also highlights some suspicious product launches that may have caused the issue, but it’s all speculation at the moment.
Apparently the only fix at the moment is to hard reset. When I better solution is found, I’ll post it here.