Console games generally tend to improve over the course of a generation as teams and studios get a better handle on the hardware and how to eke the best results out of it.
There are limits, of course, and this Reach trailer far surpasses the 360's limitations. As DigitalFoundry said, the trailer is likely rendered at a very high resolution with maxed-out AA/AF, then later scaled down to 720p. And there also may well be post-processing effects (film grain, additional blurring on edges) that will not be present in gameplay. Simply put, the clarity and crispness on the edges of things (there is no aliasing present, virtually none), and also the clarity and crispness of dynamic lighting/shadowing, is impossible on this or any other current-gen console, and on most PCs.
That's not to say Reach won't look better than Halo 3 (God, I'd be disgusted if it didn't), but it will not, indeed can not, look as good as it does in this "in-engine" cutscene.