In reply to cahotic:
The local service in Valdres:
http://jvb.no these guys also operate the beltebiler (cars on caterpillar tracks) in this part of the Valdres.
The other bus is :
http://www.valdresekspressen.no These guys can take your from Oslo centre to Beitostølen or Tynnkrysset.
www.nsb.no run the trains from Oslo to for example Otta (you get off here to go to Lom).
You can take a bus right up to Beitostølen and then ski in from there or take a beltebiler from there into Bygdin hotel. The road over Valdresflya is normally reopened after winter before easter. JVB operate a bus thereafter. There is a Youth hostel at the top of the mountain pass:
http://www.valdresflya.com/vf_main_no.html should be open as the people are trying to keep it open most of the year now. Great place for day trips.
You can take a bus right up to Tynnkrysset (This is the small town before you go over Filerfjell. You can get in a beltebil into Fondsbu or bus if the road is cleared. There are a few option there now. 1) DNT hytta, 2) Eidsbugarden hotel (new owners) 3) camp. This end of the Jotunheim is really beautiful. There is also another lodge on the way to Fondsbu.
Some of the heartland hytta (Gliterheim and Leirvassbu Mountain Lodge) both operate beltebiler, but only around Easter.
The start of May, the Jotunheim around Krossbu will also be open, and even the road over to Turtagrø. It will be possible to get a bus up from Lom to Krossbu. Really great ski mountains here. Storebjørn is a must!
Its been a weird winter this year. In the west huge amounts of snow and with us in Valdres not so much. Temps have been up and down. When planning a tour in early May I would avoid destinations that require crossing large waters, like Gjende. We undertook a tour a few years back. We first crossed Gjende but on the way back after 7 days the ice was to dangerous and we had to walk out carrying all our ski gear. Simple waters that could be crossed quickly had now long detours. Not fun.
Fondsbu, Krossbu and the Hurrungane these a good for May.