The value of a certificate of achievment depends on the repuatation of the certificating authority (university, training company etc.) and the perception by the user of the certificate (potential employer) that it is fit for purpose. A certificate basically says that the holder has met the requirements of the certifier. When a potential employer is looking at your CV, they know nothing about you as an individual. Relevant certification can provide useful information for the decision making process.
in reply to Expert programmer's certification
It is increasingly popular for the software vendors to run an education / training scheme that may result in a certificate. Attending a training course (often expensive and paid for by an employer) is a good indication that an individual has received instruction in a particular subject. If they pass an exam, then they have at least been paying attention during the course. A potential employer would probably give some weight to a candidate that had received vendor training.
Another tactic is that software vendors publish an accreditation scheme and then get companies who focus on training to run the scheme. An example would be if you took the Sun Java training course from QA Training and passed the exam. The Java course is set by the vendor and QA is a well known and respected UK training group. This combination looks good on a CV.
The limitation with training of any type is that if you don't use it, you lose it. I have known people who collect the vendor certifications but without practical experience, any qualification just demonstrates that when you took the exam you had enough information lodged in your short term memory to pass the test.
I have a lovely cerificate from a University because I did a degree in Chemistry. Unfortunately I can't remember a thing about it!