So I was just testing out this rolling thing in Sonic Rush, and yeah, CrazyTails is right. If you want to speed-up while rolling in Rush (and Sonic 4), you MUST hold down right or left while rolling. If you don't, Sonic just rolls slowly.
I tested it on a hill, and Sonic was able to go faster down the hill by rolling instead of running. But ONLY if I was holding right on the D-pad.
I also tested on flat horizontal surfaces. Rolling worked as it should if you hold down right/left on the D-pad.
Well at least it's in the game. But still, it's pretty annoying. :| What's with all this weird momentum-losing stuff??