1. You have to pay $18 or $36. A one time $36 activation fee still applies to 3G plans, the way it did for the first generation iPhone. If you're a current AT&T subscriber, this fee is only $18.
2. Data costs more. On the original iPhone plans, unlimited data cost $20/month. The new iPhone 3G plans cost $30/month.
3. Exchange costs $15/month more beyond that. The normal 3G iPhone data plan costs $30/month. Enterprise data costs $45/month. This buys you access to all the standard Exchange features. If you need to get Exchange mail, or access data inside your firewall, you're an enterprise user. AT&T isn't saying they'll disable ActiveSync and Exchange on normal iPhone plans for non-corporate users so we're unsure how they'll police this policy. Our Christina speculates that the enterprise plan sets up the server to accept exchange support; you could do IMAP from your exchange account but if you want non-MobileMe push, you gotta be on the enterprise tier plan.
Follow the jumps for more plan pricing details.