Linux to GPLv3: A practical matter not a legal one
If Linus Torvalds and the other key kernel developers were to back a shift to GPLv3, and if there were reasonable legal air cover from respected open-source authorities for doing so, it seems unlikely that we'd see a substantive challenge to such a move.
A couple weeks ago, the Linux Foundation released a podcast interview with Linus Torvalds that, among other topics, touched on if or when Linux would "upgrade" from the GPLv2 license to the new GPLv3 version that was approved last year after much acrimony. Allison Randal's summary over at O'Reilly Radar seems about right: "In the end, what we have is a stable system by reason of inertia. It may eventually shift, but not anytime soon."
One major reason is that Linus just doesn't see any compelling reason to make the shift--which is to say that he doesn't see any particular advantage to Linux at this time. That said, he's indicated the relicensing of OpenSolaris under GPLv3 could persuade him to make such a move. (Sun Microsystems floated such a move as sort of a trial balloon but it was shelved--at least for a bit--because of vocal objections from the OpenSolaris community.) In other words, if shifting Linux to GPLv3 would give Linux access to code that it wouldn't otherwise be able to use, that could be interesting. On GPLv3--as on many other matters--Linus is a rather pragmatic individual. Truth be told, far more pragmatic than many of the other highly visible personalities around open source.
Thus, don't expect Linus to push Linux toward the GPLv3 for philosophical reasons. Indeed, he's objected loudly to the GPLv3--although his more recent rhetoric has calmed--precisely because it essentially makes philosophical judgements about allowable uses (especially Digital Rights Management) that have nothing directly to do with code.
However, in the wake of this interview, a meme was also making the rounds that it would be difficult from a practical and legal perspective to move Linux over to the GPLv3. At issue is that Linux (as in the Linux kernel) is licensed under GPLv2. Some GPL’d software is licensed under the terms “GPLv2 or later,” but not Linux itself. Furthermore, contributors to Linux do not assign their copyrights to some other controlling entity--as do, for example, contributors to the Free Software Foundation’s GNU project. Thus, the logic goes, relicensing Linux under GPLv3 would require getting agreement from hundreds of contributors or more--and, perhaps, even having to rewrite code submitted by people who don't agree to the shift or who couldn't be contacted.
I'm not a lawyer and have no legal opinion on this, but I wanted to point out that Eben Moglen discussed this situation at the Red Hat Summit last May. While certainly not a definitive opinion, as the former general counsel for the Free Software Foundation that created the GPL, Eben's voice surely carries some weight. As I noted in a piece I wrote at the time:
From Eben’s perspective, “My guess is that Linux is a collective work…as evidenced by a decade of LKML [the Linux Kernel Mailing List] discussions. That’s my guess.” So, while it remains very much an open question whether Linus (and the other lead kernel developers) would want to make the move to GPLv3, it’s unclear that there are any fundamental roadblocks (such as having to get explicit agreement from every person and organization who ever contributed to Linux) should he choose to do so.
To be sure, the answers to legal questions are often ambiguous; as Eben also noted, there are alternative theories that could play here as well. However, if Linus and the other key kernel developers were to back a shift to GPLv3, and if there were reasonable legal air cover from respected Open Source authorities for doing so, it seems unlikely that we'd see a substantive challenge to such a move. Oh sure, there would be loud hand-wringing over at Debian and in other forums--this is open source after all. But, as a practical matter, I'd expect any "controversy" to blow through pretty quickly.