from late breaking news 4.1:
http://manuals.info.apple.com/en/DVD_Studio_Pro_4.1_lbn_z.pdf
"Asset Information
This section contains information about using assets in DVD Studio Pro. Encoding H.264 Assets for Use in HD DVD Projects When you export a movie from Final Cut Pro or Motion and want to use the H.264 format, you must use the H.264 for HD DVD presets included with Compressor to ensure the video is compatible with DVD Studio Pro. Do not export through QuickTime.
If you make a custom H.264 preset in Compressor, select the Multi-pass setting and set the bit rate to18 Mbps or less to ensure DVD compatibility. Currently, only progressive H.264 video formats are supported, including 480p (NTSC only), 576p (PAL only), and 720p (NTSC and PAL). You can add 1080i HD video to an HD DVD project by using the HD MPEG-2 encoder (either in Compressor or in DVD Studio Pro). "
so i guess its HD MPEG-2 for now for 1080i... bummer