I'm dead jealous!
I went in 2001 and am desperate to go back.
I only had a crappy film p and s back then and it just didn't cut it (and nor did the wally holding it).
I'd definately take a tripod.
The best light for mountain shots is always in the early morning or evening.You'll need to stop your lense down a bit to get a good depth of field and will want your lowest iso setting to get as much detail as possible so you're going to have long exposure times.
A polarising filter if you want to shoot in the middle of the day.
A fast wide angle lens for interiors. Nepal is full of little temples and if you're on the trail the lodges and local houses are all interesting too. Not to mention the people in them! None of these places are well lit and flash would just look wrong. This would be a good landscape lens too.
If weight's not a problem I'd take a longer lens. The Nepalese are fairly shy and don't always appreciate foreigners shoving cameras in their faces and there are always lots of good telephoto shots in mountain scenery.
If you go to the wild life parks in the south of Nepal you'll want a long lens there aswell.
I'm hoping to go back in a year or so with a 350xt and hopefully will take my flimsy tripod, a Canon 17-85 IS, a 70-200f4, a 50f1.8 (it's so light) and a yet-to-be-decided wide lens. I go hiking and carry my own gear though so this lot really hits the weight limit. I'd take a longer lens and a really solid tripod if I could (and could afford them :sad
Have fun. It's a truly beautiful country.