I am more and more leaning toward the Talisa is a spy theory.
The letter
The guy who created the Valyrian language had this to say about the letter:
http://www.dothraki.com/2013/05/gryves-se-rina-litse/
"Regarding the letter, the text of it was written by Cat Taylor (Dave and Dan’s assistant) and translated by me. The shot of it is quite pretty; the art department did an awesome job! Ideally it should be in Valyrian glyphs, but I guess it didn’t seem worthwhile to create an entire writing system for what ultimately is kind of a throwaway shot. Though I do have the text of it (in both English and High Valyrian, which is what it’s written in), I don’t think I should put it up right now. I’ll put it up when the season’s done with, but there’s been a lot of creativity amongst fans regarding Talisa, and so I think this should remain a mystery for the time being. It isn’t gibberish, though, I can assure you."
Him and a bunch of people are discussed the letter in Latin to avoid spoilers.
But the fact they are that coy about the contents of the letter speaks volumes.