Illuminations Birthday Cruise is a special pontoon, only 1 of five, that has HB banners tied to the sides and accomodates 12 guests. I've read that it is the hardest ressie to book in WDW since there is only one cruise each night and only on 1 boat. I've booked it in June for DH and had no trouble booking (it was much easier than Cindy's) but I did get up and call exactly at the time they opened. There is a spot on DIS boards that lists the info as follows:
Birthday Illuminations Cruise is held on a pontoon boat for up to 12 people. The boat is decorated with streamers and balloons, and they include a cake, party favors and cold beverages (non alcoholic). You take a cruise around Crescent Lake, then into Epcot for Illuminations. The cost is $175 plus tax (up to 7 guests), or $230 plus tax (8-12 guests).
They require a credit card number but do not charge unless you fail to cancel within the time limit or after you take the cruise. You get to choose the cake flavor,filling and the writing on top so it is somewhat personalized.