Answer is 3. 70-236 to earn the MCTS; then 70-237 AND 70-238 to earn MCITP: Enterprise Messaging Administrator. We are updating the prep guides to say this clearly - as shown below in 238 example...
70-238 Credit Toward Certification: When you pass Exam 70-238 Pro: Deploying Messaging Solutions with Microsoft Exchange Server 2007, in conjunction with passing Exam 70-237 Pro: Deploying Messaging Solutions with Microsoft Exchange Server 1007, and Exam 70-236 TS: Exchange Server 2007, Configuring, you complete the requirements for the following certification:
Microsoft Certified IT Professional: Enterprise Messaging Administrator
You can good info in our February Live Meeting on the new exchange certifications.