Adobe After Effects is a much better tool for doing zooms and pans over a layer.. it has precise control and does a very good job of creating smooth movement. Premiere classically didn't have the same kind of subpixel interpolation as After Effects, so you could never get the smoothness and the image quaility, but I think the new Premiere 5 can do nicer subpixel interpolation than Premiere 4 so there might actually be some plug in out there.
Neither of them will do a good job on an image at greater than 100 percent resolution, I guess God hasn't told us how to do that yet. After Effects is also very good with Illustrator/EPS files in this use.