What is the technical benefit of that? Does that skip the compression?
Yes, it will skip the compression although it will compress the signal to Cineform I assume. But still usable only then if D90 could output a clear image, without metadata, via HDMI. If not the clean 1080i image can be achieved via firmware update at least as the HDMI output supports ZOOM in etc at full 1080i rez. Check the D300 specs or samples about HDMI functionality, I assume it will be close to what D90 will have.
T