It cuts both ways. It's both good and bad. Yes, corporate ownership is a great thing, and it speaks well that companies such as IBM, Sun, Google, and Oracle show interest in open source. It may help suit and tie wearers to understand that open source != hobby quality software. But on the down side, if big company decides that it's roadmap for former open source project is where it's going, regardless of the desires of the users, well it could sour people on the product pretty quick. Even though it's open source still, the product could be forced don a path it's users don't want. Replace the community with a pair of corporate blinders and it's a problem.
Sure you can fork and all that jazz. Nothing is the end really, but corporate acquisition can be a boon or a thorn for people that just want to use a product. Depending on the product, your user base may be mostly "users" anyway. I'm no expert, but I'd imagine *successfully* forking something like MySQL isn't something you could just do overnight. There's way more to forking than just checking in the code.