Same concern. AT&T's check through text message isn't accurate, as it's always delayed by a few hours (dial *3282#).. ...and it's confusing me. I'm not sure how much I used yesterday, but I think it was only 5 megs. It MAY have been 90 though. It's really not clear. (Prorated data plans make reading this information harder. I have an "Unlimited Basic" and a "200mb plan" listed.)
I used to have a fantastic app for the iphone that did this - it was called 'Download Meter.' I've asked the developers to bring it over, but it's unlikely.