Not sure it matters, since your hd tv has to upscale it anyway.
… is the correct answer, it’s largely marketing. Your DVD player puts out ~600 lines of information (give or take 25 lines for PAL / NTSC) and your shiny HD TV will display 720 or 1040 lines; without some form of video upscaling, you’d end up with a display surrounded on all sides by a good deal of black space. This is true whether the player is a Blu-ray player or a DVD player, you’re still going from a DVD source to a HD display so the same processing has to occur.
That said, the scaling engine may be of a higher quality in a BD player than in your TV. There’s a lot of willy-waving from manufacturers about their various video processing engines, though whether or not you believe a heavily ‘enhanced’ image is a good thing or not I’ll leave as an exercise for the reader.
So in short, do you need to worry about upscaling? No. Should you let the player do the heavy lifting rather than the TV? Probably.