The last official word I head was they are "considering it", back last winter... but nothing since then.

I still hope they will add it someday, hopefully with a neat "Spycraft" themed DLC that every character can play through, set all over the knopwn maps as well as a "neutral city" somewhere (think cold war era vienna, perhaps... isle of stirk redone for this, maybe?), with a "loyalty called in question" plotline, that has a big choice in the middle - stay loyal to your alliance and prove your innocence, catch the real culprit, etc.; or defect to greener pastures, grab some secrets on the way out and evade the agents after you... bonus points for featuring fomr known faces (Who wouldn't want to slam a porticullis shut into Razum-dars face as you hop on a boat bount towards Morrowind with Naryu?)

Possibly with some nifty titles depending on your choice: "[original faction] Defector" for those who run, and "[faction] Loyalist" for those who stay...

And maybe also with some PvP drawbacks, to reassure those fearing PvP-imbalances, like earning only half AP as defector (since noone trusts a traitor, not even those in the goal of their defection), but being worth extra AP if killed by your original faction, possibly even with an "hunt traitor" daily quest that will highlight defectors in cyrodil...

And possibly also an highly overpriced service token in the crown store that pops characters over to a new alliance for those people who just want to PvP (yeah, let them pay through their noses, those PvP-swin... uhm... swits. )