I'd recommend booking as soon as you find a cruise, room and rate that you are interested in. Availability definitely goes down as you get closer to the date, and prices generally go up. We booked our Dream cruise on opening morning in Nov 2009, and our category had sold out by mid-morning. Bookings opened up yesterday for Gold and Platinum cruisers, and today for everyone else, and Disney's 1st ever cruise to Hawaii sailing 4/29/12 sold out before 2pm today.
At to travel agents, I've booked directly with Disney, AAA, Dreams Unlimited, Mouse Ear Vacations (aka cruisingco.com) and Travel On A Dream. After booking a couple of trips on my own, I use a travel agent for the convenience of having someone do the legwork for me, and for the onboard credits, gift cards and other treats they provide. (I was glad it was my agent sitting on the phone with DCL for 4 hours yesterday and not me!)
Tracy Whipple at Travel on a Dream (
[email protected]) has all of my current Disney bookings (5 cruises and a Disney vacation!) and I recommend her. Pam Schuck at Mouse Ear Vacations is my 2nd recommendation, and the folks at All Seas Travel also get good feedback.