Can't see the point of an HDTV set-top digibox without an HDTV display.
Although, don't see why a downscaler would be a significant problem, but it would have to interlace too. I would imagine that it would be built in as standard to allow for tv in other room much like Sky digital does; and I it is not likely to be a second HDTV.
I can only see a problem if the HDTV is displaying 24fps material.
Although the most commonlly discussed HTDV formats are USA's 1920x1080/30p and 1280x720/30p the European versions will be 1920x1080/25p and 1280x720/25p; but both will support 1920x1080/24p and 1280x720/24p for movie playback.
Note:
US versions support 30p, 29.97p, 24p, 23.976p, 60i, 59.94i
European versions support 25p, 24p, 50i
* I'm sure that display devices will play content from which ever version.
We'll still have equivalent of NTSC vs PAL even in HDTV -- except for movies at 24p.
StooMonster