GPLv3

Version 3 of the GPL is in draft form
Final envisioned for early next year

Relatively unpopular in kernel circles
Anti-DRM provisions in particular

The kernel is explicitly licensed under GPLv2
No default shift to v3

Changing the license would be hard
Hundreds of copyright holders
Achieving a consensus would be hard
Simply finding everybody would be hard

The end result:
A license change is unlikely