I am using iPhone 4, iOS 5.0, Currently on Maxis's Value Plus Internet 80.
And the weird thing is
I found out my data usage increase with Wifi on.
My room have wifi on 24/7, and I can still online when I woke up

Yesterday before I slept I check my GPRS Usage with *100# (short cut key is 6-1-7) and it showed
44.72mb as at 11:30 16-12-11

But I just woke up and check again
45.12Mb as of 5:35 17-12-11
increase 0.4Mb in 5 hours itself.

Today I am not going to use any 3G related apps /function, gonna see how much it used up again, cause I notice my usage increase quite a lot!

Last time I called and asked maxis customer services before. It cost more to terminate than just wait for the contract finished.