umop-apisdn is right about the quality problems with transcoding PAL to NTSC. NTSC is 30 frames per second while PAL is only 25. Visualize those 25 frames on a timeline of one second. Now slice up the second into 30 frames and move those 25 in. The missing five frames have to be created by software, and the result can suck.

This guy has an interesting write-up about this topic:
http://www.michaeldvd.com.au/Article.../PALvsNTSC.asp