Feedback on free software licence

[Tristan Miller]

Greetings.

I have a colleague who wishes to release a program under a free software
licence.  However, he and/or our company wishes to reserve the right to
use and relicense any third-party contributions under another (possibly
non-free) licence.  I therefore suggested using the following licence:

Gnomovision is released under the GNU General Public License, with the 
additional proviso that for any modifications to the source code which you 
publish, you agree to grant Vice, Inc. a perpetual, world-wide, 
royalty-free, non-exclusive license to use, reproduce, modify, display, 
perform, sublicence, and distribute said modifications (or portions 
thereof) either on an unmodified basis, or with other modifications, as 
source code or binary code or as part of a larger work.

However, I believe that such a licence would break compatibility with the
GPL (and probably most other free software licences).  Am I correct?  If
so, I suppose this might discourage people from contributing to the
project, and would also preclude the inclusion of or linking to
GPL-covered third-party code.

If anyone has comments on this licence or ideas for modifications or for
some alternative licence which could be used, please let me know and I'll
pass them along to my colleague.

Regards,
Tristan

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