Just made my first 16:9 spot. It was a really simple deal, 2 video shots of a talent read, then a full screen graphic. The whole spot is like only 3 cuts. So the on camera potion of the talent worked ok; I captured in FCP5 as DV NTSC Anamorphic (off of an XL-2). Edited the spot in an Anamorphic sequence. Created a 5 layer full screen graphic, using Title 3D, and editied that into the Anamorphic sequence. Then, for final output for broadcast, I created a new 4:3 sequence and copy and pasted the Anamorphic sequence into the 4:3 sequence. The on camera shots lost a LOT of quality and sharpness, I'm guessing that's from the anamorphic to non-anamorphic conversion render, and all 5 layers of the Title 3D were all shifted vertically out of place! I had to re-postion all 5 layers in the graphic!
Did I miss some sort of setting or something in the conversion process? Or is this just the way FCP works? Should I expect the quality loss when converting to 4:3 in video shots all the time? Am I going to have to re-do all my graphics everytime I go to 4:3 from 16:9? I hope not...

I'm notsure why it happened but i would suggest rendering out a Final Cut movie then dropping that into a 4:3 sequence and seeing what happens.