So what was the main thing you turned off to help?
I just spent a day going through everything - all the location and system services, one by one.
Headline is this:
iCloud (with all its switches off) consumes 3G data all the time wifi is unavailable.
It can be around 2k every 10 seconds. Easily reaching 20MB over a typical day with typical time away from WiFi.
The only way to stop 3G data consumption is to delete the icloud account from the phone.
This is with 5.0.1 on an iPhone 3GS 16MB. (Black :)
This took a day to test because I switched off everything else one by one and it had no effect, until I deleted the icloud account. So I removed all the location services, I deleted a load of my subscribed calendars, I removed the Reminders that were linked to my email account. I took off everything I could manage and there was no effect.
I have an experiment that is really easy to reproduce Wherever you are:
Switch off wifi so your phone is only using 3G
Go to Settings, General, Usage, Cellular Usage
Press Reset Statistics to get cellular data sent and received to zero
Go back a screen
Count to ten
Press Cellular Usage again
Look at the stats now and it will have gone up by anywhere from 1 to 20 kBytes
Repeat this by going back a screen, waiting, and returning to the cellular usage screen.
Each time your data usage will go up by that amount.
I tried this out for a long time, switching something else off each time, and usually switching the phone off and on too, to ensure the thing I switched took effect.
I left icloud till the end, though switching off the icoud settings was the first set of tests followed by switching off all the location and system services.
It was only when I deleted the icloud account from the phone that the data consumption stopped. I left it then for minutes and got no further increase. Leaving it longer caused some increase due to checking mail over the air, but nothing like the rate with icloud on.
I then added icloud back to my phone and ran the test again. Data consumption as back up to its high level and only stopped when I deleted the icloud account again.
I'm in no doubt now that having icloud signed in on the phone is what's caused my 3G data usage to rocket ever since I installed iOS5 (and 5.0.1)
If anyone has any further insight into this, or even if you have repeated my test above, with same or different results, then please do let me know.