Did they give you a business card after the interview with their email address? If so, I would send them a second follow-up letter in the form of an e-mail. In this way, you can thank them for the interview and the time on the phone when you called to check on the decision making process. And then you can reiterate your qualifications stressing the points that would bring you and/or keep you on the top of the heap.

Ummmm, I know this kinda b!tchy and that you know to do this anyway, but I am a bit of a grammer and spelling brat so be sure to either use the spell check feature on your email before you send it or proofread it thouroghly if you don' t have that feature. Like an idiot, I thought that I was sending a perfectly grammatical and correctly spelled letter and when I read it over after sending it, I found two spelling errors. And one was in my name. DUH!!!!!!