What are the restrictions of the GPL?

Speaking generally, the GPL places no restrictions on use, or internal modification of code. You can take a GPL release and use it all you want, including with modifications, without issue.

The only time the GPL comes into play, is when you attempt to distribute a derivative work based on a GPL's product - If you distribute the work, then you must 1) provide source code of the modified work, including all components that link to it, and 2) accept that anyone else gains this same license.

For the practical case, You could take MTOS, bundle it with all the proprietary code you want, and run it, without any issue. You are under no obligation to distribute your proprietary code, because the GPL doesn't cover program output (i.e., the actual site you serve), but the code itself, so unless you want to distribute MTOS with some proprietary code, there is no issue.

If, however, you intend to distribute a modified version of MTOS code, that code must be distributed with source, and your users gain all the rights of the GPL to your code (including selling it also).

Plugins are a different story. the GPL doesn't consider "mere aggregation" of content as a derivative work, so if you distribute your plugins with unmodified MTOS code, you wouldn't be under obligation to release your plugin as GPL.

You may want to just read the GPL itself. It's a surprisingly easy read.

Apparently the forum software just silently converts any links to underlined text. Great.

Since there's also no way to edit previous posts, here's what I was linking in the previous reply: http://code.sixapart.com/trac/movabletype/browser/branches/release-29/COPYING

Another forum bug (I'll just keep posting them here since I can't create a new thread): "Last post" column on forum frontpage shows incorrect dates.