Alright, I'm entering the fray, as I just got back from college to find one waiting for me.
Mine is an 8x10, arrived via USPS with no return address or post mark, just the barcode, my address, and two first class stamps.
Mine is also personalized "I

Mike" in a different marker than the signature and in the same handwriting as the envelope, so I'm assuming secretarial personalization.
I am not an expert in Taylor Swift autographs (or determining preprints for that matter), so I do not know if the autograph is sec, PP, or authentic presigned.
All I know is that the personalization was added by a different person at a different time.
Strangely, mine doesn't match the MO of the other successes (large photo arriving via FedEx, now unpersonalized), but the only person who would have known to send this to me would be the person I spoke to with her management.
So, if nothing else, I can definitely vouch for the fact that her management is sending out photos bearing a signature of Taylor Swift.
Unless Taylor wrote out my envelope personally (that was a joke), the personalization is sec. The signature, though, is still debatable.
To everyone who will PM me for the email address I used (yes, I did email): Do what I did. Google her management company. Find their phone number. Call them and ask the receptionist to please help you out. If you can't afford the long distance phone call or whatever, I'm sorry, you're out of luck as far as I'm concerned.
To everyone who will berate me for saying the above: It's common sense. If I thought to do it, other people will to.
ADDENDUM: I just looked up the PP letters with the sec personalization. My personalization matches those, which confirms my beliefs in that regard.
ADDENDUM #2: My signature does not exactly match any example of Taylor's signature that I've seen, though if it is a sec, it is a different sec than the one that personalized it.