Someone emailed me the other day asking how to loop video playback in FCP's timeline. After thinking about it for a little bit (and way over-thinking how to do it) it hit me and it's simple.
Set your In and Out points in the timeline. Don't bother with Option + A to select the portion of your sequence within the In and Out points. Tap Control + L to turn on Loop Playback, then hit Shift + \ (backslash) to just play the portion of the sequence between the In and Out points.
(Thanks to Josh for bringing this up!)