I'm making a game with similar movement for my rotary compo game
By using this expression:
X = centerX + sin(a/pi)*radius
Y = centerY + cos(a/pi)*radius
Where a is the period of the movement
You get this:
<img src="http://i.imgur.com/uzv1U.jpg" border="0" />
World.X/Y is the center of the level
Speed (global var) is speed + playerX (which is changed by pressing left/right)
Height (global var) is height + playerY (or can be a fixed value, for constant distance)