Not long ago, I posted a three-part series explaining the MCITP exam and what you need to do to start down the road of obtaining yours. The link in the sentence above takes you to Part II which explains the exams needed.

Microsoft actually offers two tracks for the MCITP. The first, the MCITP: Server Administrator is designed for mid-level administrators, while the MCITP: Enterprise Administrator is the highest-level certification designed for top-level administrators. You're responsible for taking more tests to get the MCITP: EA exam than the MCITP: SA exam.

The other major difference is that the "final" exam, which is a job-skills exam, is diferent between the two certs. Until now, that final exam in either track was not available. These exams, 70-646 and 70-647, were released last week, so it is now possible to fully obain the MCITP.

Be careful in the exam you choose to take. Taking one does not work towards the other's certification. From the Microsoft certification blog:

Is Exam 70-647 a superset of Exam 70-646? I.e. if I take Exam 70-647, will I earn MCITP: Server Administrator certification automatically? No. In the past, MCSA could be a subset of MCSE, but based on feedback we received, we now have two separate paths: Enterprise Administrator is focused on design and Server Administrator is focused on operations. Unlike MCSA and MCSE certifications, these are two different job roles with two different sets of requirements. If you earn the MCITP: Enterprise Administrator certification, you do not automatically earn the MCITP: Server Administrator certification.