If you decide to open source at all, don't go hybrid on the license or the edition (free/enterprise).

Use something like the Affero GPL to make sure people hosting the code contribute back all changes, but don't use a confusing license, or you'll keep away the guys you'd like to have contributing to your code, and only attract those that won't respect your license.

If you have an enterprise edition (with AD support, for instance), what will you do when one of your users comes up with a patch to implement it in the free license? If you want to maintain the comunitity leadership, refusing features will make that difficult.