OpenGL is the 3D API SL
uses to render (display) its 3D graphics from its propietary 3D engine
...as far as I can tell there is unanamous approval of OpenGL over DirectX within the LL developer pool. Primarily for portability, but also for helping to maintain sanity. I don't expect we'll be switching to DirectX soon.
(Andrew Linden, 8/6/05