My cateye computer has been recording my mileage (on all my bikes) in miles since I got it, or more accurately, since I reset it in Jan 2009.
I'm off on a touring holiday to Holland and it would make sense to change it to measuring in km's.
Ny question is: will the odometer carry on recording my total mileage, converting what I've done already into km, and then when I return from the land of tall blonde liberal people (for shure!) will it switch back to miles when I change the units? Or will the poor thing be more confused than I am?

I'd imagine it'll do it automatically.

Failing that, most modern bike computers will allow you to manually set the odometer so just keep a record of the mileage (or KM) that you have done.