While there are always flukes in any set of statistics, and you may end up with a shutter that lasts half as long or twice as long as its rated lifetime, generally speaking they should last for as long as they are rated. First off, they are computed to be statistically accurate. You should take a shutter actuation rating at face value. A shutter rated for 300,000 actuations came from a particularly good batch, or a batch manufactured with more stringent specifications. A shutter rated for 100,000 actuations came from a batch that failed at around 100,000 actuations during testing, which is a pretty average number for most entry and mid level DSLR cameras. A high number of shutter samples from a batch are tested in continuous tests until they fail, and the average failure rate from the whole set of sample shutters is computed. Shutter actuation counts are computed in the same way. Instead, they perform an accelerated test on a high sample count of hard drives by putting them under high stress, and compute an average, statistically useful MTBF based on the failure times of all the drives in the test. If a company tried to, they would spend some 11 years testing out hard drives that needed to have an MTBF of 100,000 hours.
It is impossible, practically speaking, to physically test a hard drive under normal usage until it physically dies enough times to actually get statistically useful results. Shutter actuation counts are computed in a similar way to hard drive "mean time before failure" ratings.
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