I've been on one cruise, to Alaska last May. We are going again in May '05. I agree with most of the packing list mail I've read, but I suggest some additions, depending on shore plans.
Your absolute most comfortable walking shoes.
10X50 binocs (the bargain Nikon at Ritz camera is super)
If you are going into the interior, temps can be below freezing, so take something to cover your ears. I bought the big fur hat with ear flaps in Carcross, Yukon territory. It was perfect. I also used it on deck in Tracy Arm. I was voted most appropriately dressed.
I'm extremely cold natured, so I wore a wool thermal undershirt as one of my layers in the Yukon.
My wife and I made one change for '05. We have replaced our water resistant wind brakers with Gortex guaranteed to keep you dry breathable unlined windbrakers with hood.
For digital camera users, you have an option on Princess, The ship will have a photo lab on board and they can download your memerory card to CD for a reasonable charge. We had 2 256MB cards and 1 64MB card and we did allright with 500+ photos at medium resolution. Our camera (Panasonic Lumix) has a 12X stabilized optical zoom which worked very well under all conditions.
I took two pair of casual slacks and shirts to mix and match at dinner, a dark suit and the rest Jeans and flannel shirts.
Above all take a ton of optimism. Even it the cruise starts badly as ours did, Alaska will work its wonders, it you let it and you witll have a wonderful time.
PS. In May you can forget the insect repellant.