I'm going to start by saying that I have never written to Marvel or emailed / tweeted anyone at Marvel re: Jean Grey. That's not me. I am simply resigned to the inevitability that the character will return.
It is no secret that Jean Grey is a popular character, and there is a population of fans pestering Marvel to bring the character back. At the Marvel panel at Comic Con 2008, it was even said that there had been discussions about bringing the character back; it was not something they wanted to rush. (Again, my feeling that it's inevitable.)
In 2007, Marvel introduced a young mutant created to resemble Jean.
The character was also given some connection to the Phoenix Force...like Jean...but not Jean.
(Mild trolling, but trolling nonetheless. Give the character black hair - or any physical features not resembling Jean, and you cut-off fanboy speculation.)
Second, Marvel is bringing back Jean Grey as a regular character in an ongoing book. Of the characters in Bendis' New X-Men, it is Jean that they put in the Marvel Now promos, again fueling speculation. Marvel acknowledges that it is Jean...but with a twist that it's pre-death Jean. (Again, that is what I would call trolling.)
Then there's the matter of that AvX 12 promo that we all assume isn't Jean (since you can't see a face), but was drawn to look like Dark Phoenix Jean.