>   So how's this for a scenario:  A company installs modified GPLed software on computers that it 
owns, but leases to 
> customers.  Have they "distributed" anything?

The licenses only require release of code, when you have modified thier
codebase or embedded it into your own code base.  If you use the
software as a helper applciation for your code, and distribute that
application bundled with  your own, you are not suddenly obligated to
release your source code the world.

Case and point would be IBM, HP, and Sun and there OSs.  They bundle,
and even change GNU utilities and complilers.  They are not required to
suddenly release the source to thier propriatary products, but they are required to
release the changes they made to the GNU utilities.

If they were to include the code of a gnu licensed product in their own
application, as contrasted with simply using the gnu licensed product
along side thier own, then they could be required to release thier
source code for thier application.

License enforcement is the issue here.  Many of the OSS community don't
have the money to enforce thier licenses, or to keep track of who might
be infringing on that license.