Minecraft's speed modifiers have no effect on players when they are not on the ground (as determined by the g:(true/false) in the debugging display). This applies to jumping, swimming, free fall, and flying (in creative).

To verify, give yourself some very high level of Slowness (like 100). When standing on the ground, you should be fully immobile. Trying to sprint will display the particles, but will not move you. Now jump while trying to move. You will immediately begin moving as normal. Slowness level 100 is an extreme-case scenario to make the problem more visible. This issue occurs with any level of slowness or swiftness.

Entering flight mode in Creative will immediately nullify the effects of the Slowness. Teleporting yourself upwards and falling down will allow you to shift sideways mid-air. Getting into water will allow you to move freely as well.

Please fix this. It's a very odd an unhelpful behavior/bug, and is causing me quite a lot of trouble.

YouTube video of this bug in action: http://youtu.be/XwhXBmGroys