Premiere does not easily handle that footage. If it's not too late, recommend they use a real video camera instead. Preferably DVCPRO HD.
So far, I'm able to edit it in an HDV timeline. I even tried to burn a segment to a standard def DVD (no menus, via encore) and it seems to work. So it seems to be working in at least a rough way.
But I still don't know how to create an HD timeline that is not HDV. For instance, what would I do with HD data from a P2 card is CS3?
If you have the last updates for CS3 (which would be v3.2.0), you'll have both P2 MXF presets, and XDCAM HD/EX presets. Check what version you're running, and if you're only on 3.0 or similar, run the Updater (Help > Updates).
You can create a custom project setting by selecting the General tab in the New Project dialog. There, under Editing Mode, you'll find an option called "Desktop" which lets you customize all parameters, including frame size, frame rate, PAR, field order and so on. Build one that matches your footage, and save it as a preset so you can go back to it later. There are quite a few threads here on the forums about editing D90 footage; a lot of them are about editing using CS4, but some are about CS3.
Thanks Colin, that's what I needed to know!
And yeah, I saw the other threads about the D90. Fortunately, I'm not encountering some of the problems others did.