I always wonder if saying interlace encoding really means anything about how the data is stored. There probably isn't any difference in the output if the decoder outputs an interlaced stream.
The reason for progressive store is that it allows the opportunity of progressive processing. But, is that any great advantage with 25p/30p?
30p has to be output as something, and that will be 60i or 60p. I doubt will see displays handling 30p inputs. So, BD goes interlaced early and HD DVD will frequently go interlaced late. Really, in the end, the display has to de-interlace. No real difference there.
And if the player outputs 60p, it must have de-interlaced it on BD and double framed from HD DVD. Avantage HD DVD there, if the BD player de-interlace isn't up to snuff. But, the player has the advantage of knowing something about the 60i stream that is missing down the pipe at the display. It knows the frame rate of the source.