I live in the US, have an iPhone4, and thusly use AT&T for data. So my experience with Bell is non-existent, and I don't know a lot about Android phones. But I have been a smartphone user since when blackberries had black & white screens, and I've had a lot of discussions with AT&T on behalf of my company's corporate accounts regarding data on smartphones, so I've been around the block on some of this topic.
That said...
Regarding that data usage that you're seeing early in the morning. It is
very likely that this is Bell's system aggregating your data from the previous business day. It is (likely) not the case that your phone actually doing something at 3am when it's turned off. AT&T's system does the same thing. This is probably a good thing, as if you received a line-item review of every data session, your phone bill might be hundreds of pages long.
The Bell rep is full of it regarding your data question. As others have said, when you're using any WiFi network, the data is flowing through the ISP that's 'driving' that WiFi hotspot. It has nothing to do with the data from Bell, and consequently
should not impact your Bell data bucket.
I don't know how exactly what options Androids have, but on an iPhone, you can explicitly turn off the cellular data. Sounds like you've got a similar option on your 'droid. With that disabled, your Android
should not be accessing Bell's data network and you
should not see any data charges (queue Mythbuster Jamie Hyneman, "Ever!"). If you do, then something isn't working correctly. Either Bell's system is screwed up, or your phone isn't obeying its settings. If this is the case, you need to talk to Bell again. Keep asking for a rep that has a clue. They exist. Somewhere.
If you've left the cellular data
on, then the phone will occasionally still use cellular data, even when connected to WiFi. Cellular data uses less power than WiFi, so as others mentioned, many smartphones (the iPhone4 and iPad 3G included), will shut down Wifi while sleeping and revert to cellular data. Like at home when you're not using the phone. I suspect this is where your data usage is coming from.
Pay close attention to oldskool's first point... There is a
significant difference between a 'Disable 3G' and 'Disable Cellular Data' setting. Disabling 3G will just revert your phone to slower cellular data (EDGE, even GPRS). The key here is that you'd still rack up the same amount of data usage, it'll just take longer. If you're in a 3G coverage area, I would leave 3G enabled.
It depends on how much email you receive and how the 'droid is managing its connections, but if you get "some" email while the phone is sleeping, then 6MB/day isn't an unreasonable amount of data.
