I patiently waited out the first couple of versions of iOS 8. After iOS 8.0.2 had been out for a while I figured it was safe to update my iPad. Not so. For iPad 2 users, iOS 8.0.2 has been causing loss of cellular data service. Obviously this only affects cellular capable iPads (3G models).
No real workaround is known. Rebooting the iPad restores service for a short time, but it soon fails again.
There is also no easy way to revert to iOS 7. So, DO NOT Upgrade to iOS 8.0.2 if you have a 3G iPad 2 and intend to use cellular data.
Apple has been slow to even acknowledge this bug, much less do anything about it.
Update Oct 13, 2014: Strange behavior — I connected my iPad to my Mac, but didn’t really do anything but transfer a picture to iPhoto. I noticed that the cellular service came back, although at one bar (I usually get 3 bars at home). It went to 3 bars, and stayed good for about a day, but is now back to “No Service”.
Update Nov 2014: This problem appears to be fixed by iOS 8.1. My iPad 2 has become glacially slow in recent updates, but that’s a separate issue.