I did 2 weeks in Kathmandu/Pokhara/Mustang valley last Christmas/New Year – great trip and it is an amazing place to visit and ride.
The trip I was on was guided by Tangi and he is definitely a good bet for finding more technical riding if that is what you are after, though he is not guiding full time AFAIK. I contacted him via Pokhara Mountain Bike club but he is based in Kathmandu and works closely with Epic bikes as above.
Pile of my photos here : http://www.flickr.com/photos/garyhill/collections/72157628783377625/%5B/url%5D
One thing to bear in mind is that many of the companies will be pitching rides at people with all sorts of levels of experience, so make sure you ask plenty of questions to find out what sort of riding is involved, particularly if you want something more technical.