I have jetwashed my mountain bike twice... will it cause any damage?
If you aim the high-pressure water jet at sensitive parts like bearings (BB, wheels, suspension, headset...) the water will get inside, and your bearings will rust and die. High-pressure water jets are also pretty good at filling your cable hoses with water, and getting past the seals on suspensions. So when using high pressure to clean a bike, stay clear of the sensitive bits, keep the nozzle like 2 feet away so it gets mist.

However if you only aim it at the frame and chain, then no problem. High pressure is really good at getting the muck out of a chain, chainring, and cassette, but it also gets the grease out, so everything needs to be lubed afterwards. Just don't aim it up close to anything that contains a ball bearing or a seal. 

As a side note, if you don't want stuff to rust... Your chain and cogs are made of steel because steel is very strong and cheap. Stainless steel is weaker, a stainless chain wouldn't last. Steel rusts though, but it needs to be in contact with oxygen to rust. The oil on your chain forms a film which protects it from oxygen (and rusting). After a good MTB ride, your chain and transmission will be full of crap and if you rode in mud, the water will have removed some of the oil. So if you can't clean your bike straight away, a quick spray of oil will protect the transmission against rust. No need to overdo it, just a little bit of oil is enough. When you clean it, this also removes the oil, so spray a little bit of it.