So to answer my original question: Just how crowded will Alta be Christmas week? It was not that crowded. A few lines here and there at both Alta and Snowbird. We never waited in a lift line for more than 5 minutes and it was mostly ski on or just a few people in a 1 minute or less line. Slopes were not overly crowded, in fact I was really surprised at the emptiness of the high traverse at Alta.
The lodges were fairly crowded and busy at lunch times. It was kind of coldish, generally 6 - 16 or so though not too windy all week. We skied 12/26 - 12/30.
Solitude and Powder Mountain had no lift lines at all... other than waiting for the bus back from Powder Country
It was an easy and fairly inexpensive last minute trip with great conditions. We will not plan this every year, but when Utah is getting the goods and we've got the time off and the means to get there, there is a good chance we will be hopping on a flight to SLC!
PS. We stayed at a Hilton Garden Inn in Sandy for a reasonable rate and they had a good hot tub and pool open 24/7 which was a nice feature. Every place we checked there had availability. We rented a basic Toyota Corolla using miles and never got enough snow to keep us from driving anywhere. Would have hopped on the bus if needed.