Thanks for the replies couple of answers:
When did it start doing this, and what did it open with before?
Actually it's been doing this for a very long time now, but has just begun to annoy me very recently (we've had an order come in which requires me to be in that dialogue an awful lot). It used to open in inches, if I recall, which is what I've got set under the Units & Rulers prefs..
You may have a British/ New Zealand/ Australia English version. Their copies do default to their metric units.
Nice guess, but it's definitely the North American version of CS1 (I'm running Mac OS10.4.7 in case anyone thinks that might matter).
As I say, I know it didn't do this at one point in time which makes it really annoying. CS1 at home doesn't do this to me, for example..
I think I'll try emailing myself a .prefs file from home to see if that cures it, but I'm sure this is one of those "hold down Option while picking something"-type tricks that I just don't know about, and somehow managed to implement one day by accident..
Further ideas still greatly appreciated!