June for Europe and ROW, I don't think they will release an unlocked version in the US as Sprint will be a 6 month (aprox) exclusive. But as with the Centro, you may be able to get the European unlocked version through Expansys (that's how I got mine). Also, this if we get lucky that they don't make deals with all GSM carriers. There are rumours that Vodafone could be the Pre carrier in Europe. But in that case, there will be unlocking services all over the internet for $20...
I purchased a Pro a week after its availability thinking the improvements would make WinMobile more tolerable. Wrong. Much better than my 750 but still locks up every other day and sloooow. Maybe the six month wait will allow some good reviews. Get the other early adopters to road test the Pre ;-)