There is several ways to convert from and to NTSC:
1.) The most simple one, which I used a couple of times and the result is pretty good: I imported the footage onto a Timeline with the settings of the original footage and exported it into the settings I wanted.
2.) The Nattress one mentioned by Johne works very well.
3.) Friends of mine did it with Compressor and it was doing a good job as well.
4.)Download the App “MPEG Streamclip” – it’s for free – also working very nicely on NTSC Conversation (A great App panyway, if you want to convert DVD Footage back to Quicktime!!!!)
Sascha