Just come back from 2 weeks in Norway. It was a camping/scenery trip with the bikes in the car for occasional rides rather than a bike trip.
Obviously we only scratche the surface in 2 weeks, but we didn't find that much classic bikey terrain. The hills are pretty steep, and amazing for walking, climbing ad scrambling, but a lot of the paths are either very easy, very touristy gravel paths and roads, or bouldery scree/morraine that's a bit Rey-ey. Or a mix of both! We stayed near Stryn for a few days, and there's apparently some good stuff near there. (And Bergen, but we didn’t spend much time round there) The stuff we were pointed towards involved a lot of very scenic, but uninspiring riding to get to some pretty good, fast steppy dowhill that was over all too soon. We did find some very nice, nadgery singletrack south of Stryn, but the rest of the path was adventuresome, in a bouldering-with-bikes, will-we-be-able-to-reverse-this-if-we-can't-find-a-way-through kind of way!
Didn't see a lot of mountain bikes in Norway. Lots of roadies though. Our conclusion was that in the areas we visited, there aren’t enough trails of the right kind and there aren’t enough snow-free, light months in the year for mountain biking to really get much momentum. Consequently, there aren’t people out there building/improving/discovering trails and it’s hard to find any info on the ones that do happen to be good.
Obviously, for the ski resorts, catering for bikes in the summer is a perfect way to bump up visitor numbers in summer, and you’re going to get gravity scenes developing round them. Maybe contact Hafjell and say you’re coming, but you’d be really interested in what xc stuff there is nearby and further afield. With any luck, you’ll get, or be put in touch with a MTB nut who’s a mine of information about trails in the rest of the country. That’s what I’d do if I went again.
And/or plan a driving/sight-seeing holiday and use the bikes as a way of seeing the country at a slower pace in places that take your fancy. Don’t get too hung up on finding epic singletrack, and expect the odd bit of hike-a-bike and adventurcising!
Have fun!