Well: I'm glad to report I now have a PDP-12. It only took three years to find one, which considering their current rarity wasn't all that long. At the moment about twenty-five machines are still known to be in existence.
Looking for it was quite a task. I contacted at least 100 used equipment companies, posted ads on Usenet, bought ads on Google, posted to various computer collectors' bulletin boards, and published ads with used equipment dealers and neurologists' newsletters (the PDP-12 had quite a following as a neurological experimentation tool). Responses led me on several wild goose chases, including a used medical equipment dealer who had foolishly dumpstered two PDP-12s in 2003, a correspondant who reported he had two PDP-12s in stock but later discovered they were Data General Novas, and one extraordinary dealer who had a clinical -12 with four expansion cabinets full of equipment who was eager to sell - but who, over the course of a year-and-a-half, proved utterly incapable of returning a phonecall or answering an e-mail (though they were always cooperative enough if they were in the office when you phoned).
Patience is a virtue, though. PDP-12 serial number 729 eventually turned up at North Carolina State University's surplus equipment store, and someone who had seen one (or perhaps several) of my ads let me know it was available. The rest was history.
As of this writing - early August, 2006 - I've just started the restoration process, documenting the hardware and preparing to restore the power supply capacitors. What challenges await after that have yet to be seen, though I suspect the LINCtape drives will provide some interesting diversions. It will likely be several months before this computer is capable of playing SPACEWAR. Still, it's a project I wouldn't willingly trade; and it'll provide excellent company for my PDP-8/I.
Unfortunately the PDP-12 came without any documentation, particularly manuals for the MAINDEC maintenance software. If you have copies - originals, photocopies, readable scans, PDF versions, whatever - of the documentation, please feel free to let me know. I'd appreciate it!