Sorry about typing 2500 instead of 2550 in the first post - I DID mean 2550 - I cut typing in High School (who'da thought I'd need it?)
Given that last night I had some time to really address this, and that I won't for while, I took the opportunity to undergo drastic measures (I was very tired of futzing with finding every driver I could and trying them - remember, this DID work,it just stopped working).
So, I reinstalled the OS, using Archive and keeping my user stuff (for laziness' sake). It's not that long a process, since I have the backup updaters to install from my own machine. It didn't work, though. I found that odd, since I'd assumed the problem to be based in the OS, as the printer worked on my backup drive, and not in my user, or my clean test user. Now, I coulda spent the next few months picking it over, but my backup was only a week old, and all my newer stuff was only a day old - AND it's been a busy week, so I hadn't done ALL that much last week. So, I backed up my immediate stuff (like email), wiped the old main hard drive, and cloned my backup to it (I use DejaVu). This morning, I moved my email accounts back in.
After repairing permissions, the printer is working great, from any program (I'd wondered if TextEdit had been the problem, since I'd been using it to test all my printing, and it had worked from the browser interface, but when testing it "stopped" on everything).
So, now I have a working system, but it took some extreme measures. My biggest disappointment is that I have no way to really know what it was that caused it - so if my clients encounter a similar issue, I can't help them fix it (I'm a Mac Consultant) - but I just couldn't afford any more down-time. Of course, I can use it as an example of the importance of having backups!
The info you've written above makes perfect sense - unfortunately, with the delay of emails in a forum like this, I've already tried all these ideas (I did so much driver shuffling - it's crazy) BTW, you should know that HP DOESN'T make their drivers for this model downloadable. They tell you it's included in the Apple OS, and that's it. What a pain! I did manage to try to scavenge them from backups, and I tried the GimpPrint (GutenPrint?) drivers, all to no avail.
There's clearly something in the communications system from the Users folders (I have to assume that they were the culprit, since the OS reinstallation didn't fix it), and maybe their ability to communicate with the OS. Also, I tried just dragging the old user folder in - but there were permission limitations, and I just didn't have the patience to try doing it as root (I'm infinitely patient on client machines, but not my own)- but that might have worked. It was actually easier to have DejaVu do all the work.
I apologize for the NOVEL I'm writing, but I know that there may be some residual curiosity on the part of anyone who's been following this. Also, MisterMe, I REALLY appreciate your help.
Take Care, y'all!