The subject of this section has been removed from World of Warcraft. As of Cataclysm, speed is dependent on riding skill, not mount speed.
The first three effects act as percent multipliers of your total mounted speed, stacking with previous multipliers. Thus, if you have spurs but nothing else, your normal mount speed will be increased to about 166.4% ((100% + 60%) * 1.04 = 166.4%). These numbers were obtained experimentally using a position-tracking addon written specifically for this purpose.
The subject of this section has been removed from World of Warcraft. As of Cataclysm all mounts are scaling mounts..
There are a few mounts available that scale in speed and flight capability based on the riders' Riding skill and location. Such mounts are rewards from holidays or outside-of-game items and are designed to be usable regardless of level or skill. That said, they only change capabilities during the action of mounting. For instance: mount up in a non-flying area of Dalaran to have a land mount, as expected. Then, ride into Krasus' Landing. The mount will not automatically turn into a flying mount. So, dismount and remount to fly away.