OK, I'll add my experience to this thread.
I've had my xv6900 since about 3 days after it became available. I waited until all of the patching and testing was completed on the Alltel ROM to do anything.
I started out by upgrading the radio to 3.37 which came with the Alltel ROM. I think used one of the existing ROMs out there, I believe it was OMJ. Anyway, once PPCKitchen updated to include the Vogue, I've been flashing my own ROMs based on that. I followed the instructions on this thread to upgrade the radio and ROM the first time:
http://forum.ppcgeeks.com/showthread...ighlight=safer
Now, a couple of observations. First, Verizon has NOT turned on EVDO. Rev A everywhere. By upgrading you may or may not get it depending on where you are. As for GPS, mine works fine as long as I prime it first using something like GPS Viewer. The only real issue I had with GPS was that, at first, I forgot to change the location service from 911 Only to location on under the phone settings.
As for the phone reauthorization thing. This happened to me right after I unlocked and upgraded the first time. I'm not sure whether it was the unlocking that caused it or the upgrading of the radio. All I know is that a simple call to #8899 from the phone and a few minutes is all it took to get corrected. I've flashed probably about 10 times since then getting my phone the way I want it and I've never had this issue again.
Feel free to ask anyting you want and I'll answer if I can.