I've always wondered if these technologies (mySQL, PHP, etc.) are going to some day be 'taken away' or something....as in 'stop being free' but I have a hard time wrapping my head around whether it's possible or if i would really care if they stopped writing mySQL or whether it's legally possible to retract my ability to use mySQL for free or whatever.
The worrying part would be the contributor agreement. In the hands of a benevolent entity, assignment of copyrights could make it easier to upgrade to a new version of an existing open source license, or possibly to deal with cases of copyright infringement. In the hands of a company that may put corporate interests first, however, things may be different. I have no legal training, but from what I understand licenses can be revoked at will, if there is no interest such as a license fee. Contracts are safer in this sense, but then contracts (e.g., AFL, OSL) have problems when it comes to typical open source distribution (e.g., no shrink wrap or click wrap).
At the moment, it seems that MySQL AB have taken the Red Hat route: allow access to the enterprise source for those with sufficient technical know how (via the version control system), force payment for more immediately usable forms of the enterprise software. It definitely is still open source software at it stands, so my concern is whether they will cut off access to the enterprise source even from version control, and/or relicense the enterprise version under a proprietary license. If they do not, then just as CentOS and other distros sprung up to be binary compatible with the official RHEL, unofficial distributions of MySQL Enterprise Server may also be made.