Howdy,
I agre with the first half of Dave's approach: Take it all apart. Clean it. Make sure the seal is straight, tight, and there's no dirt on the sealing area. While most leaks are user error, its good to know that Eheim's 3 year warranty will take care of any hardware issues. Before you go to your LFS, call the Eheim service hotline. Unless, of course, you bought your Eheim at your LFS. Then these guys should be able to take care of it free of charge.
But I disagree with taylor. I had the pro 1 (same head seal), and the seal never leaked...
HarleyK