It's not going to be easy, or maybe even possible to know for sure what amount of text messages are diverted to iOS vs SMS/MMS, because you have multiple groups that are putting together their pieces of the puzzle. Apple only knows how many iMessages have traversed their network, but they don't know how many SMS text messages were sent because that's something the individual cell network carriers do separate from Apple. And I'll bet you the cell carriers aren't interested in sharing any data that highlights a situation where their SMS traffic (and revenue) have dropped.
Likewise, Apple probably isn't keeping track of who is on which carrier, so it's impossible for say, AT&T (as opposed to Verizon or Sprint, or Orange or Vodaphone or Softbank) to know for sure what slice of that iMessage pie would've gone through their network had iMessage not existed.
So, all we can gather is from personal experiences, which are gonna vary by a lot. Some people have lots of friends and co-workers on iOS 5 using iMessage, while others not so much.
I'll say this much: before iMessage I was sending roughly 800-1,000 texts per month on average and had the unlimited messaging plan. Last month? I was billed for 53, and at the causal rate of $.25 per message on casual texting rates, that would be $6.25 less than unlimited messaging. And I know my texting habits haven't decreased any, all of that unaccounted-for traffic went through iMessage.