Most of the major lines have the own towers now, but as stated above, expect to pay fairly large roaming charges as you're on a private tower, owned by the cruise line. They make money when you talk from their ships! (I don't care what your contract says, "Nationwide Free Roaming" usually doesn't cover private or foreign countries' towers.)
Now for the good news: Most of the Inside Passage (SouthEast Alaska) is in the process of being covered. There are new towers in Angoon and Elfin Cove that weren't there summer-'08.
I'm picking up service better than ever in Frederick Sound. I used to have to stand on the heli-pad at 5-Finger Light to get 2 bars of service, now I can call almost anywhere in Frederick Sound. There are dead spots near Tracy Arm and between Cape Fanshaw and Petersburg, but I'm sure with the new cruise ship development at Hobart Bay that will change soon as well.
AT&T just bought out our local Cellular One franchise, and the crews are coming to the area NOW to change service over and add - you guessed it - MORE TOWERS!
Anyone on Cingular or AT&T should be in good shape for 75%+ of your cruise in '08/'09 and beyond. Anyone with another carrier that has a roaming agreement with AT&T should be okay as well.
Oh, and just in case you're wondering, my iPhone (with AT&T service) works brilliantly within hundreds of miles of Juneau.
Having said all that, get a pair of the FRS (Family Radio Service) walkie-talkies. Pick a channel besides #1, and agree to leave them on. When I want to lose my DW and just hang out in the bar, I just turn mine off for an hour or two and suffer the consequences when I come back to our stateroom! ;-)
Better not for now... signatures get me banned!