Not so, says Drysc. By decree of the devs, the biggest mount speed bonus items can grant will be 10%, which means the Riding Crop does not stack with any other items (keep that in mind before you spend a ton of gold on it). It's meant, says Drysc, as a space and time saver more than anything else-- don't carry around three items when you can carry around one.
Of course, as alert players point out, there are class speed buffs that will stack with Riding Crop-- Crusader Aura is one for Pallies, and I'm sure commenters to this post could come up with more (I thought that Aspect of the Pack would, except that AotP doesn't stack with mounts at all, so of course it doesn't stack with the Riding Crop). There are probably even those among you who wonder why any more than a 10% buff even matters for getting from place to place, but for some players (especially those in battlegrounds), every second spent on a mount counts.