In the past, a company caught violating the GPL was able to be reasoned with via letters and ultimately came into compliance. While the threat of legal action has always been there, it has served as enough of a deterrent to keep companies in check.

Not anymore. Now, for the first time ever in the U.S., a company is being sued for a GPL violation over the use of the open source project BusyBox. Monsoon Multimedia, who manufacture among other things the Hava media box, have stated that they do use the BusyBox software to help run their devices. After conversing with the company, the developers of BusyBox were apparently unable to convince them to come into compliance:

The developers of BusyBox came to the SFLC after trying to talk Monsoon into honoring the conditions of the GPLv2. Unsuccessful with this, the SFLC has filed suit on the developers' behalf against Monsoon.
They do still assert that their goal is not monetary compensation, but to see the company come into compliance. That would be a very simple thing to do, you would think, and would cost a lot less than being taken to court. Nothing has been heard yet from Monsoon themselves.