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It is possible to constrain an object to a path using parenting [ctrl+p>>Follow Path] and bypass use of the constraint altogether. The constraint exists for bones, since bones cannot be parented to objects outside of the armature. And so the constraint can be used for both objects as well as bones... producing two methods for following paths which can be a little confusing.
Without use of the constraint, follow path options are set in the curve object options. When using the constraint the 'Curve Path' button must be enabled on the curve itself, but 'Curve Follow' is determined by the button in the constraint options, not in the curve. The speed for both methods can be controlled by adjusting the 'path len' property in the curve options, or in the 'path' IPO.
To use the Path IPO select the curve object, set the IPO type to 'path', then ctrl+LMB to set a key frame. The length of the path is represented by a range of 0 to 1 in the Y axis, and the timeline is represented by the X axis (remember in 2.4x the timeline starts at 1!).
Options and Settings
OB: The Curve Object to serve as the path for the constraint.Curve Follow Determines whether or not the constrained object or bone will follow the orientations of the curve.
Offset: Determines the value of the first frame.
Fw X/Y/Z(-) Up:X/Y/Z Determine the 'forward' and 'up' axes of the constrained bone or object. These can enable you to re-orient the object or bone on the curve. The constraint will turn red if an impossible value is entered (for example X/X, Y/Y, or Z/Z, since a single axis cannot be both the forward and up axis). Influence Determines the influence of the constraint as a percentage from 0-100%. This value can be animated manually, or driven by another object or bone in the IPO.
Show Shows the constraint in the IPO window.
Key Sets a keyframe on the current Influence of the constraint at the current frame.
More Resources:
Blender Wiki Page - Follow Path
