Here is a FAQ from the GNU site. I think it answers your question:
Does the GPL allow me to sell copies
of the program for money?
Yes, the GPL allows everyone to do this. The right to sell copies is part
of the definition of free software.
Except in one special situation, there
is no limit on what price you can
charge. (The one exception is the
required written offer to provide
source code that must accompany
binary-only release.)
http://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLAllowMoney
Further more:
If I distribute GPL'd software for a fee, am I required to also make it
available to the public without a
charge?
No. However, if someone pays your fee
and gets a copy, the GPL gives them
the freedom to release it to the
public, with or without a fee. For
example, someone could pay your fee,
and then put her copy on a web site
for the general public.
