Have had experience with this recently - I've found that rejections come faster, it may be a situation in which they love you and one other person, or they need some kind of approval from corporate, something like that.
I can relate to your search. Although I am employed, I am in the process of moving on elsewhere, and HR or hiring committees anywhere are like a huge black box, you have no clue what's going on. I've had some strange situations where they dragged it out, claiming they loved me the entire time, for two months, and then decided not to hire anyone (didn't get approval to bring someone on into their particular group, budgetary issues - that's the assumption, anyways). I've also not heard from a place for three weeks despite follow-up, and then magically, a 2nd interview. I think the economy is making companies extra cautious - which is really frustrating for job candidates!
Please keep us posted and don't give up hope! Also, following up more than once is fair, but at least a week apart, IMHO - and if you wrote to ask the first time, perhaps call the next (or vice versa). I've done it, I felt awkward and pushy, but it ended up working in my favor in at least two instances (I was so surprised because I honestly thought they'd think I was annoying/neurotic, but it seemed to actually be a positive...).
My POV is based only on recent experience looking for a job in my field, but I felt I'd love to share because the whole job search process can really wear anyone down and I've been there :/
Best of luck!!