+(copied from my response to
two earlier posts from today on the same topic)+
It's tied to Settings > General > International > Region Format. If that's set to the UK, for example, Maps uses the metric units (km), if set to the US, Maps uses 'English' units (miles). I know, that comes off like a bad pun (the UK not using English units) but that's the way it is...
If you can live with the week starting on a Sunday in the calendar month view, and with the US date and phone number formats, you can leave it set to the US.
You can submit feedback to Apple:
http://www.apple.com/feedback/iphone.html