My setup is a fairly standard one:
13″ 2013 MacBook Air, which is occasionally, although more often than not, plugged in to a 27″ Thunderbolt Display.
I “upgraded” to Yosemite last night.
Booted my machine, everything seemed fine. I was downstairs at the time, no Thunderbolt display connected.
This morning, plug in to the TB display, and disaster… Blacked out.
Machine restarts saying it restarted because of a problem. One off I thought….
Then, a few seconds later, same thing.
I unplugged the TB display, and it booted fine.
Plugged the TB display back in, it crashed again. I could replicate and reproduce this every time.
Secondly, if the TB display was left plugged in, then boot the machine, the problem would happen, and it would get stuck in a cycle of booting, crashing, and restarting itself.