Joint author doctrine

[Justin Wells]

What does joint author doctrine mean for free software licenses? I 
read this document:

  http://www.macleoddixon.com/04public/Articles/04artpub.html#intell

which includes the following quotes:

    The Copyright Act provides for joint authorship when a work is
    prepared by more than one author "with the intention that their
    contributions be merged into inseparable or interdependent
    parts of a unitary whole".

    ...


    1. Each co-author will own an equal ownership share in the
    work.  This will occur even if one of the co-authors has
    contributed a greater quantity of the work than the other
    co-authors.

    ...


    3. Any co-author, without the permission of their fellow
    co-authors, may grant non-exclusive rights to the work to third
    parties. However, a co-author may only grant exclusive rights
    to the work to third parties if the co-author obtains the prior
    consent of the other co-authors.


So it sounds to me like if you allow someone to collaborate with you to 
write some software they become a joint author of the resulting work. Does
this apply to all people who release derivative works with your permission?
Are thay all automatically joint authors with you over the result?

What's worse is that if this is true anyone who contributes to your project
can get around the GPL and grant non-exclusive rights to the whole work to 
anyone they like. 

So I could contribute a few lines of code to the Linux kernel, become a 
joint author, and then sell non-exclusive rights to it to Microsoft?!

But I'm not a lawyer, and now I'm feeling a bit lost. Someone help me figure
out how this applies to the GPL and other opensource licenses.

Justin