GPL question

[Lorentz]

Hi,

I've written a web-based system in Java that allows people to sort of
create their own functionality with my API. It integrates their work
(which is basically an applet but can work with and extend the existing
bits of theprogram) and stores it on my server.  

I definitely want the "drops" people submit to be GPL'd and also the ones
I write myself.  This way the usefulness of the application will grow
quickly.  

But I wonder how the GPL treats the application framework itself.  It's
an applet helped out with beans/servlets.  If I release that part under
the GPL and post the code, what does that allow people to do with it?

They may take it and run the same service I am.  But they aren't really
"distributing" the program--only providing a service with it.  Would they
be required to post any changes they make to the code?

Let me know if I've given enough info to illustrate my problem.  

Bottom line: I don't want someone to take my code if it turns out they
don't have to release under the GPL and they still get to offer the 
functionality to the public. 

Thanks for your thoughts.

L