In actuality, as Dana Blankenhorn points out, MySQL started moving away from pure open source last year, when they created a split between the Community and Enterprise editions, and delivered closed-source software as pay-only services from their web site. This split recognized that they serve two different customer basis (as Zack Urlocker said just today): those who wish to pay, and those who wish not to pay. To my mind, there's nothing wrong with this hybrid approach! Indeed, it's the same approach that we've taken since our inception 3 years ago, and that we refined when we launched Postgres Plus last month.
I do believe it is fair, however, to ask for transparency and clear language. In Zack's blog, he wrote, "...just to be clear: (1) MySQL 6.0 backup capabilities are open source, (2) Add-on modules will be for paying customers." Perhaps I'm splitting hairs, but I'd rather see #2 say "commercially licensed" or "closed source." That's what it is, and there's nothing wrong with it.