On PowerVR MBX it is implemented as two triangles. However I’m not sure why you ask this question. Regardless of how it is implemented, rendering behaviour should be the same, since that is what the spec dictates.
PS: My target device is a Nokia N95 @ PoweVR MBX + that V-somethign extra accelerator thingy (probably hw T&L or so, don’t remember exactly)
PowerVR VGP, and yes, it’s hardware T&L (and more flexible vertex shading, which is not exposed on any Nokia device).