She has tweeted that she responds to fan mail, but I wouldn't really recommend sending anything to her to be signed, because I have been waiting over a year to hear back from her, and know others have as well. Good rule of thumb-- that I forgot in this instance-- is to never send something you'd be sad about not getting back.
BUT if you are just sending a letter, there really is no risk and it's of course worth a shot for the $1.50 it costs in postage :)