I am still trying to understand where these triangles are being formed.
object starting point is red
object ending point is pink
light grey lines are examples of nonrelative x and y offsets.
black are examples of directions of motion.
when you're trying to get the speed relative to an angle, you're trying to get the black part of the triangles.
<img src="http://dl.dropbox.com/u/1013446/spriterspam/xyrel.png" border="0" />