Certificate in data mining I am interested about getting a certificate in data mining. I found three so far, and these are the following:  


MIT - Data and Models in Engineering, Science, and Business
Stanford - Data Mining and Applications Graduate Certificate
UCSD - Data Mining Certificate


Has anyone taken any of these courses, and are they worth it? Would I benefit in terms of career opportunities with one of such certificates?  I already have a background in data mining, however nothing is on paper, so I thought it might be good idea to get a certificate.
As with any certificate course, it really depends on what you intend on getting out of the program. All of those are well-respected universities, and the coursework will almost certainly be extremely informative. In that regard, you'll learn a lot. On the other hand, there exists a frankly ridiculous amount of high-quality, freely available information online related to data mining from many universities, including some of those you listed above, so you can definitely learn all that without paying a not-insignificant sum for the experience.

Regarding job prospects, it really depends on where you're looking to be hired. This answer from Workplace.SE is an excellent overview of how useful a certificate can be in helping you get hired, and this question from Programmers.SE has many useful answers discussing how certificates like the ones you mention are viewed in industry. Long story short, they're usually not that useful for jobs unless they're industry standards, which the ones you list are not.