Compatible of MIT License Patch with GPLed Software Project

[Yan Cheng Cheok]

Hi,

I have an open source project which is copyrighted under my name and licensed under GPL2.

In order to incorporate work with other contributors, I follow some policy found in http://www.virtualbox.org/wiki/Contributor_information

(1) I require the contributor to sign an agreement http://www.virtualbox.org/wiki/ICA, so that their code change is jointly copyright owned by the contributors and me. Then I will license the changes under GPL, as I am the copyright holder too.

Recently, there are new contributor who refuse to go to step (1), they prefer to release their changes under MIT license.

According to the statement in Virtual Box site

"If you don't want to sign such an agreement, you can alternatively submit your contribution under the MIT license. This is a liberal, wide-spread Open Source license that allows Sun (and anyone else) to use your contribution in both open-source and closed-source projects."

(a) What I understand MIT license is that they are not copyleft protected. Does that mean, I can released the changes under GPL license? But I am not the copyright holder, how I can do so?

(b) Once I have a software project which is "GPLed + copyrighted by me", combined with some other changed which is "MIT licensed + copyrighted owned by others", do I still own the right, to redistributed the whole projects under other license in the future?

Thanks and Regards
Yan Cheng Cheok