Won't iCloud eat up all your date if you are on a limited data plan?
Possibly. That's what WiFi is for. The biggies will be music and photos - which I would think would mostly be synched on occasion, could always be done from home/work/coffee shop with wifi. The other stuff, which you are more likely to sync "on the go" like calendar and contacts are nits, size wise.
I was on the beta for iCloud and the data usage was only 4-6MB more. This was without Photo Stream, Reminders or Notes synced. That being said you probably won't have many problems - plus I believe Photo Stream is over Wi-Fi only (at least that's what it says in the Settings app - never tried Photo Stream)
So far what I've been hearing is that the Photo Stream and music backups should be used only on WiFi networks, and if you are careful about doing that a normal 2GB/month data plan will suffice nearly anyone for their data usage. iCloud is pervasive throughout the OS and the internet capabilities of the device are obviously an integral part of making that work. From what I know, there is no reason to be worried about having to pay over-cap rates for data once installing the new OS and making use of iCloud.