Thanks for all of your suggestions. We shall be doing some research now...

I must put in a good word for skiing in Colorado.
I may be biased because I live there but some of the best Skiing in the US is right here.
Vail and Aspen are nice but could prove a little expensive for a family trip.
I would recommend Breckenridge . Big resort lots of terrain over four peaks.
Good for kids, excellent ski school. Lots of gentle greens for the novice, lots of blue for the more experienced skier.
The town is nice. Its an old mining town with that old west feel to it.
Ski passes for Breckenridge also work at Keystone which is another good family resort and only 9 miles away.
Lots of buses run between them.
We have skied these resorts for the last 3 years running.
If you like the sound of this and want more info let me know.