GPL and software timeout/expiration

[Paul Pang]

Dear Sir/Madam,

I am considering starting a new project that would 
leverage the source code of an existing GPL software. 
My software would be released under the GPL with
source code available for download and original 
copyright notices. To cover my development costs 
(the GPL portion is a very small part of the 
software) I am planning on charging customers. I 
understand that I can sell the binary software online
(http://www.gnu.org/philosophy/selling.html) so long
as I comply with the GPL license and make the source
code available freely.

What is not clear to me is how I can charge for the
software. What I have seen is people charging upfront
then making the binary available for download (an 
example of this is Phat Linux.) In my case I would
like the software to timeout after a period of time 
(say thirty days) after which the user would have to
pay a reasonable fee to register it. I would like to
know whether this complies with the GPL or not.

Best regards,

-Pang Paul

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