The parameter you need to change is SPEED, which is available in the right-click menu for clips on the timeline. To go slower, speed <100%. To go backwards, speed <0.
When PPro CS3 comes out, it will be possible to keyframe speed, but in all current versions of Premiere a single clip can only have a single speed.
Another way to change clip speed is to use the Rate Stretch Tool to change clip length by dragging ends on the timeline.