Based on one of the PPs comments, it would appear that it is not a matter of whether or not Disney has plans to sail in Alaska, because if they didn't plan on it, they wouldn't have put in a proposal. The question would be WHEN?
DH and I sailed on HAL's Stadendam in 2000 from Vancouver BC to Anchorage. It was GORGEOUS!! Ketichan was an absolutely adorable little town, and we did the Inside Passage, so we also stopped in Juneau, toured Glacier Bay, stopped in Valdez and also Sitka. We ended in the port (I can't remember the name right now) that has the train into Anchorage. My parents took the same boat, but did it southbound, and instead of Sitka they stopped in Seward. I would say if a family were to do an Alaska cruise, it would be best to go on
DCL, exactly because of the reasons people said. Kids may not be as excited about going to Alaska, but will be well entertained on DCL vs a HAL ship, for example.
Yes, airfare is costly, but its costly just to fly right now, period. Anyone who has ever wanted to cruise Alaksa wants to do Alaska, no matter what the ship (though some are definately rumored to be much better than others). If a person has no desire to cruise Alaska, they probably wouldn't cruise Alaska on DCL either. If they have always wanted to do Alaska, and love Disney, what a great opportunity!! My DH has no interest in the Med. Cruise, but that doesn't mean there aren't a billion other people who would love to do a Med DCL cruise. (including myself...its not cool to go without your DH is it?)
I'm totally excited

about this, and I have already been on the Alaska trip. There are plenty of other excursions to try (ours weren't that expensive, BTW, but you could feasibly spend close to $1000 for one helicopter excursion in one port too!) Now I am trying to figure out how many DVC points I will need to bank and borrow to get on that boat!!