you can go to teh Ski Club of Great Britain website.
At teh end of the Northern Hemisphere ski season tehy publish a list of ski resort close dates but I can't find an equivelant opening dates list.
You can find out resort by resort (worldwide locations), the official opening dates (when all lifts and hotels should be open if there is sufficient snow).
But you have to go to them one by one.
For example here is the Resort Intro page for Arosa 28th November.
www.skiclub.co.uk/skiclub/skiresorts/resortintro.aspx/Arosa
then just click on the resort drop down box for other resorts.
Also provides ofther info such as altitude, current snow conditions, cost of ski pass etc.
Cheers,
Higgy Baby