iPhone data usage broken down by app

I know this doesn't answer the question specifically, but are you sure you are on wi-fi when its available and not just solely on 3G? I've made that switch in the past and forgotten to go back. Also, have you installed any apps recently around the time the spike occrured?

Onavo (available via iTunes) is a great app to monitor and breakdown your usages, it displays data usage by app and compressed data over 3G.

EDIT: The primary goal of this app is to reduce data volume by routing all traffic through Onavo's servers and compressing the data between the phone and the server. Depending on your privacy needs this may not be acceptable for you.