MiniDV's native resolution is:
NTSC - 720x480 pixels, 29.97 frames per second
PAL - 720x576 (or 570, can't remember) pixels, 25 frames per second
When you send your MiniDV video across S-Video (or composite), it is transmitted at 720x486 (the extra 6 lines are added to the top/bottom by the camera). Also, for reference, true 480i is defined as 720x486 at 29.97 fps.
When you send MiniDV out over FireWire, you're simply transmitting the 720x480 compressed frames at 3.5 megabytes per second, with no frame rate or size conversions being performed.
The protocols used for sending MiniDV over FireWire are completely different than the new 5C standards being pushed by Sony for HD. Just because it is using FireWire doesn't mean it's sending 1080i data.
That being said, I'm assuming that any of the new HD gear that includes 5C/FireWire ports will accept standard MiniDV streams, upconverting them accordingly, but I haven't come across a set to try this with yet.