// And for that matter, I'm not a license expert either
Unless the copyright is assigned to you, the original author maintains the copyright on the code.
The simplest approach to this is to have contributors license the code submission with a BSD license. This license allows you to (with some strings attached) to incorporate the code into a compiled commercial product (you don't need to release your private contributions).
A reasonably thorough analysis of the BSD license for open source (your community edition) can be found at http://www.freebsd.org/doc/en/articles/bsdl-gpl/article.html
What is necessary (remember that 'IANAL' bit) for the modern BSD licenses is to simply say that it use it in your application's about information. An example of this in an installer.
As long as they are licensing the submissions to you with the BSD license (or compatible one), you can then use it.
You may also wish to look at the MIT license.
As I understand it (which could be wrong), the difficulty with using an (L)GPL license for the contributions would be that they would need to be in a different part of the application that you would link to - a library - but not part of the core application.
Your desire to be able to deliver binary distributions (presumably without the source) would make anything that is GPL licensed difficult to use.
Going back to the original author, none of this prevents the original author from licensing the contribution again in another product.
If your community edition software is BSD licensed, there's nothing to say someone couldn't fork it into GPL and then start contributing to that, with the licensing of the contributions being GPL (and then you wouldn't be able to use that without releasing the source). You may wish to find something that prevents the software from being licensed or forked into an open product that prevents you from reincorporating the contributions back into your closed version. The advertising clause in the original BSD license would prevent it from getting foked into the GPL.
If you are concerned about the GPL forking possibility, another such open-but-anti-GPL license is the Microsoft Public License (Ms-PL).
If in doubt, or confused, talk to a lawyer.