After much pain, I realized that when I turned fast switching off, printing to the 950 started working again as normal. Not sure why this is, but it appears that when using fast switching, the sharing of the communication to the printer suffers. At least the the 950 is now usable, but this may be something for Apple to fix.
[robg adds: I haven't had any issues with our Epspon inkjet or Brother laser and fast user switching, so I'm not sure if this is local to one user or not -- if anyone can confirm (or not) this one, please post a comment...]

