Found this in my e-mail this morning. It was for my wife's account who hasn't played since late Wrath. Checked my account page after this and it was already upgraded to Mists standard edition. Didn't have to activate anything. Not sure how many of these are being sent out and to who, but thought I'd share since I didn't see it mention anywhere.
(edit) To be clear, you don't have to renew your account to get it, if you see this e-mail the expansion has already been added and you can renew your account at any time afterwards to play it.