A Word About Shutter Ratings
Shutter actuation counts are computed in a similar way to hard drive "mean time before failure" ratings. 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. 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. 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.
Shutter actuation counts are computed in the same way. 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. 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 shutter rated for 300,000 actuations came from a particularly good batch, or a batch manufactured with more stringent specifications.
Face Value
You should take a shutter actuation rating at face value. First off, they are computed to be statistically accurate. 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. Second, statistical ratings like MTBF or shutter actuation counts tend to be pessimistic, rather than optimistic. I guess this alleviates legal pressure from those who like to sue over petty things, and gives more weight to the rating.
Usage and Enviromnent
Your personal experience with shutter lifetime may be dependent upon how you use it. If you put an entry-level DSLR through professional-grade usage, with continuous burst shots of sports or wildlife action in rugged, dirty, or moist environments, then you could quite likely get consistently less use out of a shutter than its rated at. On the other hand, if you put a highe end professional camera through more mild use and take extremely good care of your gear, the shutter could last considerably longer than its rating. A shutter rated for 300,000 actuations (i.e. the ones used in a Canon EOS-1Ds MkIII series body) is not only manufactured to a higher spec, but tested by continually activating a sample until the shutter fails. At worst, you may continually activate a shutter for a couple hours at a time, after which the camera is stored until its next use.
If you are looking into buying a camera with a lower shutter rating, say 20,000, make sure that you understand how you intend to use it. I bought my first camera ever, a Canon Rebel XSi (450D) about 18 months ago. I burned through 5000 shots in the first two months with it, and have over 10,000 shots on it so far. I expect this body to last me at least a few years, and if it only had a rating of 20,000 shutter actuations, it would probably be done by the end of this year. When it comes to most DSLR's, I think the lowest shutter rating is 100,000, so generally its not a problem. I can't say for P&S cams, though, but I would say that a shutter rating of 5000 is not going to go very far at all, and something along the lines of 20,000 actuations would be more reasonable. This assumes it has a shutter...many P&S cameras use electronic shutters.
