Some aspects I'd iCloud do use 3G data so you must be careful. My wife has the 200mb plan as well but in one night her phone used 130mb for something. I then changed a few settings to see if I can curb the midnight 3G data usage that the phone does. 1. Settings > iCloud > Documents & Data > Use Cellular OFF (this was most likely the culprit but we don't know for sure). 2. Settings > iCloud > Storage & Backup > iCloud Backup OFF (this prolly isn't needed but I'm paranoid with her going over the 200mb). 3. Settings > General > About > Diagnostics & Usage > Don't Send.
There are many threads on Apple's own discussion forums with ios 5 using 3G data in the middle of the night even though wifi is also connected. Since we also experienced this with my wife's phone we are being extra cautious.