Not directly related, but I did have a situation like this where I shot a video with a Canon T1i at 1920 x 1080, not realizing it was limited to 20fps at that resolution. Imagine my shock when I imported the file and noticed how juddery it was! Fortunately the clip in question was very low-motion (shot of a talking head, done handheld but pretty smoothly), and I was able to use the Optical Flow image retiming in After Effects to boost it artificially to 24fps (didn’t feel like pushing my luck to 29.97). It ended up working out—the few weirdo motion artifacts were so minor as to be unnoticeable in realtime, and no one said a word.
Depending on the nature of your clip (and, obviously, if you have After Effects) this might be a good way to go.
The initial convert is probably best done in QuickTime Pro, honestly. Dave, how is MPEG Streamclip for duties like this? I’ve never used it as a general purpose video converter, just for situations where I needed to go from an MPEG-2 source.