(c) permission on multi-authored works [was: Charging the Charger]

A few posts ago, I promised to look into a legal question about permissions 
on multi-authored works.  Checking with an IP law scholar provided a pretty 
good clear answer. Context follows:

At 01:21 AM 4/11/2005, Joe Corneli wrote:
   At 23:56 4/10/2005, mva wrote:

>    All someone could do would be to charge for the increment, the part
>    he/she added.
>
>One potential problem would be - say that the two of us co-author a
>program under the GPL, then I go AWOL.  I don't know the nitty-gritty
>of the copyright law, but it appears to me that unless there was some
>assignment of copyright made, I still have the copyright in my
>portions of the code, and you in yours (even if these are inextricably
>intermingled).  If this is right, then without my consent, you
>couldn't use the new clause.

I posed the following questions to a friend/colleague who is the author of 
"Digital Copyright" a very good text on (c) issues.

>If two people co-author a copyrightable work, does a downstream licensee 
>need permission from only one or from both?
>
>The question may also be subtler for different works.  For a coauthored 
>article, the contributions of the 2 parties may be 
>indistinguishable.  But, the reason I ask is that in the context of (open 
>source) software, the larger whole may have modules that are authored by 
>different parties.  The parts may also have readily identifiable authors.
>
>I can see how the GPL handles this by granting default rights to 
>*everyone* to use, modify, and redistribute.  But, as a general rule, how 
>do you navigate the permissions necessary to use a co-authored work?

She replied:
Only one [needs to grant permission].  Either author can license the 
work.  If I write the lyrics and you write the music to a song, either of 
us can license the song, you can license the lyrics (even though I wrote 
all of them and you wrote none of them) and I can license the music.  We 
have a duty to account to each other for any proceeds.
So, opportunities for licensing are quite flexible. Hope this helps clarify.

MVA