I have had a set of Mavic XC 717 Rims with 36 spokes laced up to XT hubs on my LHT since I built it up in the summer of 08. When they were first built the valve hole was machined a little rough and I had to file it down myself and then put some extra rim tape around it which solved the problem.
Around the 2,000km mark the front hub started making a lot of strange noise and there was an issue with the bearings and caps or something, and I had to have the hub body replaced (this was done while I was in NZ and I wasn't entirely sure what the actual issue was, but this is what I remember being told by the mechanic).
I have put about 7,500km on them, mostly on pavement, but around 500km on gravel/dirt roads, and maybe 50 of light mountain bike trails.
Love the rims, and apart from the front hub I have had no problems. I feel confident riding on beat up back roads and decent MTB trails if I am not too heavily loaded.