For what it's worth, over here in Australia, we've had MNP for about 2 years. It hasn't significantly increased churn -- in fact, after all the hype by the Government about being able to change carriers on 1st Sep(? I think - it was something like that) that in the first 12 hours only 3 people, australia wide, had actually changed. It's a reasonably common thing now, however. I haven't bothered with keeping my number, as I enjoy getting a new phone number every year or so, but my Girlfriend has kept her number from Optus (which was an 0402 or 0412 prefix) to Vodaphone (which were 0414, 0415 and a couple of others).

--Rob