Nodes, not miners. (?)
From what I read, the threshold is 75% of the previous 1000 blocks mined by XT-compatible miners, but in January 2016 at the earliest
Kinda,
The trigger to allow larger blocks is dependant solely on miners.
Quote
Activation is achieved when 750 of 1,000 consecutive blocks in the best chain have a version number with bits 3 and 14 set (0x20000004 in hex). The activation time will be the timestamp of the 750'th block plus a two week (1,209,600 second) grace period to give any remaining miners or services time to upgrade to support larger blocks. If a supermajority is achieved more than two weeks before 2016-01-11 00:00:00 UTC, the activation time will be 2016-01-11 00:00:00 UTC.
Of course, as we've seen recently, the rules that the miners pretend to follow, and the rules they actually follow, are two different things.
Also, if the blocks created by the miners are not accepted by a similar "supermajority" of the clients, then those rewards from creating the blocks will be difficult to spend.
It's gonna be a big, hot, mess.
Add into that prorogation time of the conflicting clients, ie if the vast majority of clients are running core and not relating bigger blocks then propagation of xt blocks would be slower and there is the chance that they wont be relayed before a core block is propogated and begun to built upon.