Development of Creative Commons licenses

[Daniel Olmedilla <olmedilla AT l3s.de>]

Dear all,

sorry if this question has already been posted but I couldn't find anything 
related in the archives.

I know that GPL is free distribution license where if a piece of software has 
this license any modification of that software must include GPL as well. I am 
interested in knowing if this applies also to applications that use that 
software.

My situation is that I am considering to use software under GPL from other 
applications and developments I make and give the whole to clients. Of 
course, the software under GPL would be released for free to the clients and 
only charge for installation (is this also possible?) but I would charge for 
the new developments (which has no lines in common with the GPL software). Is 
that possible?

Some examples:
- Use of an existing GPL Web System like PHP-Nuke or PHP-Post and develop 
specific modules/plugins to fit a company (without touching at all the GPL 
sofware, only separate modules) and charge the client only for the modules.
- Use of one of the existing research P2P networks (e.g. Edutella) and build 
an application on top of it (the application is a client of the P2P network 
and they are actually different programs).

I know that GNU allows you to sell any product that uses the software under 
the GNU license (e.g. Apache Axis), right? However, I am not sure if it is 
possible to do something similar with GPL.

My problem is that some companies have many requirements that are already in 
existing products but they need some extra modules/apps which are not. One 
solution would be to develop everything from scratch but then clients would 
have to pay a lot for that. The other possibility is to charge clients only 
for the new developments (e.g. they install themselves a GPL software) and 
ask a company to develop some extensions for them (and pay for that). Is this 
possible?
- If yes, which are the restrictions? Of course the client must be perfectly 
informed of which part is GPL and which part are extra modules.
- If not, which possibilities are to do this kind of things?

Thanks in advance for your help,

        Daniel