Does iPad use data when turned off?

OK, I'm a bit confused. On August 23rd, I purchased the 250MB plan for my iPad. I checked email regularly, browsed the web occasionally, and tried out a GPS app. On September 10th, after 18 days of use, I got an email alerting me that I had 20% of my plan remaining. So September 11th, I purchased the 2GB plan. Yesterday at 11:27am (9 days after purchasing the 2GB plan) I got the 20% remaining alert while sitting at a workshop with the iPad off. At 11:41am I got the 10% remaining alert.

After getting the 10% alert, I immediately turned data service off. Here's where I'm confused. I didn't think iPad supported multitasking (at this time) so unless I had an app running, data would not be being used. Is this wrong? Can iPad still receive data when turned off (clicked off with button, not turned completely off)?

While I was in the workshop, there was no app running, the main screen with the icons was displayed when I switched iPad off. How was I still using data???

Thanks...