I've mentioned this before, but if you're serious about taking the exam, the Exam Aids (coupled with the exam prep guide from Adobe) are the best resources to prepare with. I know its an extra cost but trust me, these prep exams can potentially a) save you time when preparing because you'll have a better grasp of what you know and don't know and, b) save you some money on retests because you'll be better prepared to take and pass the test the first time.
Now, I realize that the Lightroom 3 Beta is out and you may be wondering why worry about the Lightroom 2 Exam. Well, if history is any indicator, its taking about 15 months or more for the current ACE exam to come out. So if you want to stay current, taking the Lightroom 2 exam is about the closest you'll come.
Here's the link to check it out.