I don't wear a suit to interviews. TBH I hate the things, they make me feel like I've been dressed by my mum for the school photo, and that's the last thing you need in your head at an interview. You should be smart, sure, but comfortable and confident. There's also an argument that if 10 identi-candidates in identi-cheap job-interview-suits turn up for an interview, the one who turns up looking more comfortable and like he's got an independant thought in his head might stand out. Possibly in a bad way, possibly in a good.
It does depend if you're applying for a "suited" position, if you want to be a merchant banker or an undertaker or a hitman or something. But I'd say myself that if the role's never going to see you in a suit, you shouldn't feel the interview needs to either.
Of course, I have a s**t job so what do I know?