> When will IBM release the source code for it and under what license?

If you downloaded the SDK drop, you already have the source. :) Eclipse is 
distributed under the CPL - a non-viral open source license that says you can 
take the code and do whatever you want with it.

> And b) Where the heck is the "project" definition stored? Only in that
> small ".classpath" file?

Not sure exactly what you're asking here. Eclipse project data is stored in 
your workspace (which is placed in your cwd by default but can be redirected 
by passing Eclipse "-data <workspace location>". The .metadata directory 
contains the state info associated with your workspace (including projects). 
So if you have projects A, B, and C, you'll find directories A, B, and C in 
your workspace folder (say /home/$USER/workspace/.). Those directories will 
contain your .java and .class files. Your .metadata directory 
(/home/$USER/workspace/.metadata) will store information like which files you 
have open, what breakpoints you have open, your view settings, etc.

> And c) What level of integration with ant is there? I read in the docs that
> it can run ant build files. But can it generate ant build files and keep
> them in sync with my whishes (project contents and dependencies)?

Eclipse only ships with the ability to run ant scripts, not generate them. 
Excellent third party opportunity. :)

- Jared