[optimistically_conservative]

http://www.fas.org/nuke/guide/usa/c3i/nuclear-football.htm

I guess it's not so much a button, but codes, and there are extreme contingencies where the nuke force commander has authority.

[July 4th]

Yes. You can't have a system where the commander-in-chief has 100% authority. The enemy just has to lop off the head, and the war is won. So in extreme cases, others have the delegated authority to launch.

Here's some info on GWEN, which appears to have been replaced by SCAMP: http://www.fas.org/nuke/guide/usa/c3i/gwen.htm