I don't know if this helps, but I have 605.3 and I've hit a solid wall of "service unavailable" and "Error 2304" messages since about 5PM last night.
It is possible that they are getting the 605.3 folks upgraded first, since they have the smaller upgrade file and can be upgraded more quickly, and they also have the Voicemail notification bug that was introduced in 605.3, but I kinda doubt that.
I think the servers are just swamped. It also happened during the 605.3 upgrade - some people had it pushed to them overnight, some got the notification followed by a connection failure followed by "no updates available" for a while (like me), then came back into queue and were re-offered the upgrade a few hours later. And a lot of people ran into a lot of "service unavailable" messages because the servers were swamped.
I expect that the problems are going to last while this time. HTC is upgrading a lot of phones at the moment. Their servers are probably severely overloaded.
Early reports of this Gingerbread build are overwhelmingly positive. It looks unlikely that Verizon will feel the need to pull this build. This looks like the build that may finally fulfill all the promise this phone has to offer. So there's no desperate rush to get this one, it should stay up on the servers until everyone is upgraded this time.
Be aware that it may be a week or two before everyone gets it, though, since the last build only reached a small number of people and was up for about 24 hours (and wasn't coinciding with major updates to several other HTC phones). In fact, if every HTC Sense phone out there gets upgraded in a two-week period, I'd be deeply surprised and impressed.
If you're in a huge rush and can handle dealing with a factory reset, the RUU is only a download away, and you'll be running the same stock firmware everyone else is (and quite probably with a lower possibility of problems since you'll get a "factory fresh" build).
Otherwise, just keep checking every few hours, and eventually you'll get your Gingerbread lovin' :-*
Note: Last time, I did a battery pull and it was very shortly after that I was offered 605.3. It was probably a coincidence (correlation is not causation), and I did it during a peroid of "No Updates Found", not during a "Service Unavailable" peroid. But hey, if you wanted to try it, it can't hurt.
Especially if you haven't restarted in a while - when you do eventually get the upgrade it might go a little cleaner if you've had a recent full reboot.