BlackBerry was already rather slow when they released x-platform BBM for iOS and Android last year. It got announced roughly the same timeframe and also for summer and it literally came the very last official days of summer. So it might be very well possible we'll have to wait until late of August till we got a version we can use and install.
While it's nice BB finally decided to bring BBM finally to Windows Phone, remember just back in January they stated there is no WP variant of BBM planned, it is mostly a problem for BlackBerry.
They want to position BBM as a leading x-platform messager and still don't have support for the now 3rd largest platform out there. Remember back when Facebook bought Whatsapp and everybody talked about it for many days? Again no action from BB to take advantage of this and promote BBM.
I am eagerly waiting too for WP support, however having been a BlackBerry user for almost (and still am actually) I simply learned not to hold my breath for any of their announced dates. They are like the old Nokia: Announce now, release many months later. Just in time everybody has forgotten about the new products and competition already reacted.