Bikesdirect.com, Nashbar.com, and a few other places will let you get a decent, serviceable bike that isn't a boat anchor. I don't see why you should get a singlespeed when you can get a fully functional 3x8 road bike for $300 or so.

I'm an advocate of cheap bikes, but that's mostly because I do all my own wrenching... take that into consideration. If you can't fix bikes and you buy a cheap one, you will make up the difference bringing it to a mechanic frequently to get adjusted.