This is going back a bit now but I thought I'd add a postscript.
It's this: freeze the damn phone, then it works.
Today I loaded the 2.33 Gingerbread RUU like I've done several times before: no dice & 7 vibes of doom.
Then I put the phone in the freezer compartment for ten mins in a plastic bag and then re-installed the 2.33 while it was very chilly.
Bingo: boots straight up, works like a charm and when I - teeth gritted - reboot it, it comes back and works like a charm again. It seems like it just gets a bit too hot and bothered installing the RUU to actually have the wherewithal to boot the damn thing without locking and 7 vibes. But if you place a cool flannel on its forehead, so to speak, it has another go at it and all is lovely. Unbelievable.
Cheers for now!