Issue with wireless data usage

For some reason, my MBP (running Lion) burns data like crazy. For example, I went from .4GB to 4GB in little over 8 hours with no use but checking emails and web surfing. No streaming video, no downloads, etc.

I am not sure what is casuing that use - maybe a background app? Any suggestions would be appreciated.

Running Lion with VZW 4G modem. It did this with 2 different modems so I don't think it's a modem issue.

 

Thanks. I'll check out Rubbernet

Found the problem - Outlook was constantly u/l and d/l to the tune of 1 MB / sec; which is about right based on VZW's data use estimates.