According to this Apple user forum discussion, if your company's Exchange server has encryption enabled on its ActiveSync settings, then anyone with the 3G model running the new 3.1 OS will find that they can no longer connect. Instead they get this error message: "The account '——-' requires encryption which is not supported on this iPhone."

Hopefully Apple will release an update to the update shortly, or your company's IT department will get all "who cares" about security and turn off encryption. In the meantime, clutch your phone dearly and keep away from any OS improvements until at least a few days have passed to reveal any hidden problems.