Sun has provided various Java certifications such as SCJP, SCJD,SCEA,SCWCD, SCBCD, etc. As you move upwards in the certification stack, the marginal utility diminishes quickly.
For an entry-level programmer, preparing for SCJP is a good way to get familiar with language basics. It also proves to potential employers that you are a motivated learner. If I were to choose from a pile of resumes, those with SCJP will certainly stand out. At this stage, they are judged more by knowledge than by experience.
For seasoned developers, their value lies in experience that is hardly testable. They may not know the details about certain API, and may even fail SCJP, but they've accumulated do's and don'ts from various successful and failed projects. They have guts feeling why certain design is better. They can smell it when some code is fishy. Because they know where to locate knowledge quickly, and how to leverage tools (such as IDE, debugger and profiler), they pay less attention to knowledge.
None of these values are easily testable in standardized format, even with some sort of sample design and essay. When applying for architect or lead developer position, certification just becomes irrelevant.