Joe: you’re definitely right about web apps. If you’re not redistributing stuff you’ve built on GPL code, you don’t have to distribute your changes.
If you’re hosting software for other folks (I can pay for Drupal hosting, for instance) the Affero GPL might apply: http://www.gnu.org/licenses/agpl-3.0.html but only if it does. Which is to say, it is a different license from the GPL.

That’s bizarre. Why would anyone ban building on GPL code?
I’d also love to know what you think about it, but I really just want to get a handle on how widespread such policies are.