I have seen this problem 3-4 times from getting the 928. Even though I have 5 bars of LTE at my home and all of the sudden, no data works, even with wi-fi, so sometimes I can go a few hours with out noticing it (as I just look at the tiles to see if I have new mail, twitter, facebook etc). Every data connection is dead. After a quick reboot of the phone everything returns back. Not sure if its changing towers, I have LTE coverage at work but, inbeween work and home, there are 3G only areas I go through. I have only noticed this at home but, I cant say for sure.
I was thinking about calling Nokia about this issue, as I could miss a important email because of this happening, and if your only just doing a quick check of your tiles, you wont even notice an issue till you try to do something else.
My software/firmware on my 928...
OS 8.0.10211.204
firmware: 1532.5957.1315.3021
it should be up to date