MySQL does a variation on this.  They have the code available under the GPL, but 
will, for a fee, license it to you in a closed source license if you wanted. 
This would allow a company to use MySQL as their db of choice without risking 
having to release their program under the GPL for using and linking to a 
modified MySQL.

> If you own the copyright and you are not including or linking to
> anything not under your copyright, this is ABSOLUTELY legal.
> 
> If you start to link or include code from elsewhere it becomes
> problematic in that you need to adhere to the license under which those
> things are released.
> 
> AOL 'kind of does this' with Netscape/Mozilla combo.  Sun kind of does
> this with the StarOffice/OpenOffice combo.  Others have tried it as
> well.  Dual licensing is not new and Im sure others will try it in
> various capacities.

>>Suppose I wanted to release a software product.  I wanted the Linux version
>>to be Open-Source, something like the GPL perhaps.  But I wanted the Windows
>>version closed-source so I could sell it.  Would this be feasible?
>>
>>Has anyone seen a license like this?