We had a similar dilemma. At our company we decided invest lots of time on a framework, with the eventual hope of releasing it to the open source community. The business is built using open source tools (apache, php, etc.), it was time to give back. We decided on an LGPL/MPL dual license. That way, we could incorporate fixes/improvements from the community, while still protecting applications (particularly ours) running on top of it from being forced to go open source as well.
