Ok so I was thinking, generally most cutie marks relate to the ponie's name. So just guessing here, Applebloom's cutie mark has to do something with her Family's Apple Orchard. At the same time though, Applebloom is very good at painting and fixing up things; such as Applejack's old treehouse. Just a theory.
As for Scootaloo, her cutie mark (as everyone has already stated) more than likely has something to do with speed and her being skilled with her scooter.
Of course Sweetie Belle is an amazing singer, so I assume just like everyone else has that she will have a music note or even possibly a microphone for her cutie mark.
Again just a theory