FYI, Those cheap HD DVD players are only 1080i.
While we're on the subject, do you notice any difference between 1080i and 1080p? I know the technical difference, and I know that 1080p is supposed to be better, but even on a 60" screen I've heard that you can hardly tell one from the other -- unless you're a salesman, of course.
The source data is stored on the disc (HD-DVD or Blu-ray) at 1080p/24 and is interlaced to 1080i/60. Your television should be able to properly deinterlace the signal to 1080p, and you will see the exact same picture. This 1080i/p thing is worthless marketing talk.
If you have a 1080 set that can play in 24Hz multiples, then you probably have the money to buy a better player that can output the native 1080p/24 signal to the set.
For the rest of us with 60Hz televisions, this 1080i issue is moot.
The XBOX HD-DVD add on is 1080p with a VGA connection.