Any travel agent can tell you what cruises are the repos. They are in the spring/fall, when ships are first going up to Alaska and back. Most are the 10 day to 2 week cruises that include the Hawiian islands.
There are also several 1-6 day repositioning cruises on the pacific side in May and September. If you go to Expedia's website, set the search criteria for May 2007, North Pacific, and sort by cruise length, it will bring up a bunch of repositioning cruises. You can do the same thing for September 2007.
Once you find one that looks good, you can call your TA or the cruise line directly for booking.
We are considering a 5 day repo in May 07.
Monarch of the Seas - December 2006
Elation - April 2006
Sensation - May 2005
There are also similar repositioning cruise from the Caribbean to Europe in April and back in October or so. Some of them can be pretty cheap - I got an offer once from Celebrity (about 6 weeks in advance of sailing) that worked out to $54/day per person for an inside cabin going from FL to London.
I like the search engine at vacationstogo dot com - you have to register but you can specify reposition cruises and see them all.
A repositioning cruise is when a ship changes home port. For instance, it may stay in the Caribbean in the winter, then go thru the canal and up to Alaska for the summer. Or over to the Med or Baltic for the summer. Most repositioning cruises (well, not the Carib - Alaska ones) involve a week or so with no ports as you cross the ocean. These cruises tend to be cheaper and they don't fill the ship. Airfare is more since you have basically two one-way flights.