We're getting a very specific error message popping up that states:
"The account 'account' requires encryption which is not supported on this iPhone."
I'm guess that our exchange server ActiveSync policy has encryption enabled and the iPhone ignored this up until 3.1. Since only the 3GS supports hardware encryption all 3G iPhones will no longer sync with Exchange that have this policy option enabled.
Seems like a mighty big 'oops' we forgot to tell you this would render your company's thousands of iPhone 3G's ActiveSync capability useless instantly.