After mountain biking in muddy conditions I clean my mountain bike with a high pressure water blaster. I'm assuming that after this I should re-lube my chain?
I've heard you should keep high pressure water away from your bike, because it can easily work it's way into the bearings, even sealed ones, especially around the bottom bracket and wheels. Pressurized water will push the oil out of the spaces between the chain links, and it's probably not a good idea to use pressurized water at all. You should be able to sufficiently clean the dirt off your bike with some light water, or, if you want to really thorough, use a chain cleaner. And you should always relube after cleaning regardless of which cleaning method you use.