Hehe... good questions 25GTT - I was actually pondering those too!
As far as I know, NOS can damage your engine and can also decrease engine life because you are putting it under a lot more stress (just like increasing boost to very high levels I guess). Having said that, getting a system installed that only gets used occasionally and with low shots of NOS you should be fine and it'd be a whole lotta fun!!! (plus if your engine goes, it should be fairly spectacular! )
Cheers,
EXIT