Bugger. While trying to get gdm working for the new Dept of Computing base install it looks like I screwed up a fair amount of stuff. That includes email from last night until now (I don't think it would have bounced - it just got vapourised), IV, comments etc.
So any important emails should be resent, please.
Oh, and the answer was that calling exec in any startup script (which is common practice here to change shells) causes gdm to fail with a completely unhelpful error message. I think we might be using kdm now.