I have searched the GPL FAQ, and found a few pieces of text that relate to this issue:

The case when someone else might possibly claim the copyright is if you are an employee or student; then the employer or the school might claim you did the job for them and that the copyright belongs to them. Whether they would have a valid claim would depend on circumstances such as the laws of the place where you live, and on your employment contract and what sort of work you do. It is best to consult a lawyer if there is any possible doubt.

If you think that the employer or school might have a claim, you can resolve the problem clearly by getting a copyright disclaimer signed by a suitably authorized officer of the company or school. (Your immediate boss or a professor is usually NOT authorized to sign such a disclaimer.)

...

We also ask individual contributors to get copyright disclaimers from their employers (if any) so that we can be sure those employers won't claim to own the contributions.

Another page has an example of such a disclamer:

You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the program, if necessary. Here is a sample; alter the names:

Yoyodyne, Inc., hereby disclaims all copyright
interest in the program `Gnomovision'
(which makes passes at compilers) written 
by James Hacker.

signature of Ty Coon, 1 April 1989
Ty Coon, President of Vice

This doesn't solve existing problems, but might help people who want to release software under the GPL.

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