Sometime Today, Anant Narayanan assembled some asciibets to say:
Safari uses a modified version of KHTML, WebKit [1]. Another example of the GPL forcing a corporate to release source code of modified GPL'ed software :)
What makes you think they were forced? Maybe that's what they always intended to do.
On the other hand, I know of a lot of small developers who use loopholes in the GPL to build proprietary applications around GPLed libraries.
I'll give you an example of an IM client (which I won't name). It uses a well known GPLed multi protocol IM library. The author wrote a thin wrapper around the library to expose all functions over a socket (TCP or unix domain), and released this under the GPL. This wrapper is so generic that it could be used for any library. He then wrote a proprietary IM application that called all functions via the socket.
A second example, also with IM clients is folks who've written web apps wrapped around GPLed code, but have no reason to release their code because they don't actually distribute it.
It's not the corporates you have to worry about. Doing something like this would give them bad PR. It's the little guys who have nothing to lose, but a bad attitude to spread that you have to worry about.