I emailed him via his studio fan mail address and in exactly 3 months, I got a letter in the mail!
edit: hmm. can't seem to attach a picture so I put it in flickr. http://www.flickr.com/photos/62078173@N00/4943933268/

Hi, does anyone know the address to the Smurf movie studio? I want to send a letter to Neil Patrick Harris and his voice for the movie is being recorded these upcoming weeks.
NPH: http://www.imdb.com/name/nm0000439/
Smurfs: http://www.imdb.com/title/tt0472181/

Thanks!