Nope, can't say that I have never gotten a response.
I once sent a letter to Phil Vischer, creator of 'VeggieTales'. He sent me back about a... 3 sentence reply, but it was pretty nice nonetheless. ^^ (I can understand too, as he probably gets swamped)
Of course, has anyone ever had an actor or the sorts actually ASK YOU to write a letter? XD That happened with me and Doug Jones about last year. For those who don't know, Dougie played Abe Sapien (the fishguy) in the movie 'Hellboy' (not the voice). He was really chuffed at my support of him and asked me to write a letter at some point.
Since then we've exchanged stuff -plus talked on the phone via a radio show he was being interviewed on-, but I've been mindful of the fact he's very busy these days. (Especially since he's starring in 'Pan's Labyrinth', I wouldn't expect a reply from my latest e-mail until filming is done) So I guess in this instance you'd say I hadn't gotten a reply, but knowing Dougie, I will within the next couple months. ^^
And hey... how many actors would be willing
to do this for a fan who couldn't make it to an event? (And for the record, I now own that particular sign)