I use Motion for Videomapping. In order to map an object it is necessary to view the canvas in fullscreen, so the canvas has the same projection as if the exported video would be shown on the beamer.
In Motion 4 this was ALMOST possible, because in fullscreen mode, there still was a thin grey edge.
But I could handle that by exporting the video in 99% scale, it than had the same size on the beamer when projekted directly as quicktime video.
In Motion 5 there is a fullscreen option in the "windows" menue, directed to F8, but why can´t you use it?
Unfortunately, this was still disabled with Lion. Maybe they are releasing updates to FCPX/Motion5 soon with that Lion functionality as well as some other stuff? One can only hope. I really hope this first update blows everyone away and redeems some of Apples shortcomings with FCPX.