Actually, DreamHost doesn't make an uptime guarantee.. I'm not sure where you may have read that, but if you look at our site, we make no mention of an uptime guarantee anywhere.
The reason we don't is simply because uptime guarantees in the shared hosting industry are generally a sham. Look at other hosts with an uptime guarantee and try and find out what the "guarantee" actually is.. IE, what happens if they don't make the uptime levels promised? Also, whose responsibility is it to monitor the uptime and decide if it reaches the level mentioned? And is it the average uptime figured over a day, a month, a year, or what?
99.9% uptime would be no more than 43 minutes down per month. ONE server problem in a month could easily put a site under the 99.9% level.
We do have our problems here of course. A day doesn't go by where at least a couple of our servers have some sort of trouble.. it's just the nature of the game when you've got over 250 machines! We always do our best though to figure out what the problem is and correct it to make sure it doesn't happen again. If there's a user with a bad script that is hogging all the system resources, we contact them and work something out. If it's a hardware problem, we always have hot spares available to fail-over a problematic web server onto. We keep all our customer websites on centralized file servers which allows us to do this. Most other shared hosts have no way to fail over a machine like that since they keep all the data on the local drive (maybe with some tape backups).
We keep our network way underutilized, and have redundant connections to different providers, so network speed is never a problem.. any downtime you've seen is most likely due to a specific server having some trouble. I promise we do our best to get problematic hardware replaced as quickly as possible.. I think most people who've had problems with their websites with us will report that we did get it fixed and since then it's been smooth sailing ever since!
Nevertheless, we do offer a 30 day money back guarantee, and although we don't make any formal uptime guarantees, we are of course willing to provide refunds and credits on a case by case basis should something go terribly wrong with your site!