Thank you! This is incredibly helpful. With the lack of feedback, I was just leaning towards doing L-R since that's probably what most users will be expecting (and I also remembered they'll be able to get to the right side quickly by just back-tabbing, first thing).
Heh, you are the third consecutive person who's asked; it should be relatively easy (most of the buttons, textboxes, etc. are all stuff Tumblr has, just in a different layout) so I'll probably put it as a setting users