In Settings -> Store I have "Use Cellular Data" disabled. However, I did an update today while connected to a WiFi network, and later saw that "Store Services" had used up 20 MBs of data over 3G in that time (this is according to DataMan.) It also used 127 MBs of WiFi data.
That's 10% of my monthly allotment! How can I prevent that in the future? Why did it do that?