This debate is ridiculous really. I just bought an Toshiba HD DVD player for $230 and I firmly believe my only risk is the player itself (and even then I have a decent upscaling DVD player). Which ever format wins the war, you can rest assured that the smart hardware manufacturers are going to support both media in their inevitable $99 players, for at least a few generations. I will not be stuck with useless HD DVD's any way you look at it.
History doesn't support this, though. There were no Beta/VHS combo units that helped Beta customers survive. Beta was dropped cold once it was dropped. There is no incentive for the winner in the war to support the "old" losing technology because you no longer have a choice but to buy the winner. IT wouldn't be cost-effective to try and support people's old already-bought movies once the war is won.
Also worth noting that some folks went for those LaserDisc players back in the day, and when that technology lost out people were left holding that bag too.
True, as long as you have your player you can play all your current stuff... but you'll have to buy a new player to play new stuff if your format loses, and it is unlikely that your future player will play your old stuff in the case of a format war.