In the very article here, the SF Conservancy claims both to have a legal basis to sue and willingness to do so, though the intention to do so only as a last resort.
In May 2012, Conservancy launched the GPL Compliance Project for Linux Developers, which handles compliance and enforcement activities on behalf of more than a dozen Linux copyright holders.
The GPL Compliance Project for Linux Developers is comprised of copyright holders in the kernel, Linux, who have contributed to Linux under its license, the GPLv2. These copyright holders have formally asked Conservancy to engage in compliance efforts for their copyrights in the Linux kernel. In addition, some developers have directly assigned their copyrights on Linux to Conservancy, so Conservancy also enforces the GPL on Linux via its own copyrights in Linux.
Although one wonders if copyright holders ever thought that they'd be suing a Linux company like Canonical...
