Suppress data/Media Storage

[msbork]

I don't know if this is coincidence, but I started having a data hogging issue ever since my failed attempt at the system update.  According to My Data Manager, something called Media Storage has used 69.5 MB in the last 13 days.  In previous cycles, this app never even appeared in My Data Manager's list.  15 MB were in a 10-minute period when I was using 1 app that, as far as I know, doesn't use media. Since I was at the beginning of my billing cycle, I pretty much stopped using the mobile internet connection because I didn't want it to happen again. I also decided to install Onavo since it can block apps from using mobile internet. 

Today I had to reboot my phone.  I didn't even unlock the screen until 20 minutes later, but in that time, the same app (according to My Data Manager) used over 50 MB. (I forgot that 3G comes on automatically when rebooting; it was turned off before I shut down.)  Onavo shows the culprit is Market Downloader, and I can't limit it to WiFi only.  I don't keep Background Data syncing turned on.  As far as I know, I've uninstalled every app that I installed recently (other than Onavo, which was installed after the first data hogging session).

Anyone know what this app is--and whether there's a way to stop it?  Or is there a way to prevent my phone from turning on mobile internet when I reboot so I don't have to remember that annoying quirk?  I'm almost ready to do a factory reset because I'd rather lose everything I have than pay for a data overage.

Thanks!