I’d probably contribute back to the company, if it was pretty much minor tweaks and/or bugfixes. For such things, it’s somewhat unlikely that they would release bugfixes and such only in the proprietary version, and I’m probably most interested in getting it to work for me.

It also depends a good deal on the license. For instance, the copyright assignment I’m required to sign for the FSF grants me back all rights of use of the code (so I can continue to do what I want with it, put it in BSD or proprietary code, whatever), and additionally promises that they will distribute the code under a Free Software license (duh).

I don’t know MySql’s agreement, but I would be really hesitant to sign any assignment that didn’t at least promise that my code will remain under GPL (in MySql’s case). Effectively barring community contributions from becoming “proprietary-only” features. That’s only fair. And if they provide that, then I have fewer qualms about helping develop even significant features. Though I still personally prefer to contribute to wholly Free (and not just Open Source) projects (in instances like this, there is, I think, a distinction).

The absence of forks (go-oo seems to be an exception) of any significance just have to be a sign that my concerns are not widely shared by open source users and hackers.