If they use some form of mpeg or something similar, they can easily support any resolution. Mpeg is basically a series of jpegs where they store the differences between frames. They throw in full frames every now and then so you don't have to play a movie from the beginning to get to say 23 minutes into it.
They should be able to scale any native size mpeg to the TVs native size using the same logic regardless of source resolution. The only potential problem is if the logic that converts the jpegs into bitmaps doesn't have enough horsepower to keep up with larger native resolutions. You see some pauses and pixellation. The kind of thing you'll sometimes see if there's an explosion (Or any scene where the pixels change rapidly) on a cheap blu-ray player.