Just remember that 5/15 (woohoo birthday!) is just the first day of the launch window. The official launch window is 5/15 - 6/3. I think it'd be an awesome sight.
I wasn't at WDW, but we were at Kennedy when they launched a shuttle and I have to say it was a VERY cool thing to see! I'll never forget it. I never thought about being able to see the lauch from WDW though. Also cool!
I saw a much smaller rocket than the space shuttle lauched one night when I was at EPCOT. This happened in either 02 or 03. I think it was putting a satellite into orbit. The launch was at 9:50 p.m. and EPCOT was nearly empty when it went up. I videotaped it and could even see when the booster rockets separated.
I hope we can see the space shuttle from where I live now, about 30 miles southwest of Disney. It should be awesome!