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Probably too late. My question is: Why didn't you call them back? They should have either told you then (When you're done a job interview ask how long they are going to take deciding) or you should have called back two days later and keep hounding until you get an answer.. >>
From being involved in hiring developers I'd say the hounding part is a very bad idea. During the interview is perfectly OK to ask when a decision will be reached, but if you didn't then call and ask
once, politely. Be a pest and you may get tossed into the discard pile.
[edit, PS] also, if you're applying at a small company there's not much value in the follow-up thank-you letter. We're all too busy doing our jobs to care

Seriously, we're taking time away from normal work to read resumes, interview, etc. and we just want to find someone good, fast, and get them on board to fill the hole in our development team, tech support, etc.