Kirk Tuck's weekly article at The Online Photographer this week was about aspect ratios, and how his preference is for the square. I think Mr. Tuck's article may have been at least partially prompted by his recent purchase of a Panasonic GH2, which has an actual multi-format sensor, although 1:1 is still a crop. I hope we see more sensors made this way.
Mike Johnston later posted a small follow-up on his preference for rectangles. In between these two articles, I also saw this post about proportions as they relate to comic book frames by Frank Santoro, via Austin Kleon. Here's a quote.
Finding the squares on a given proportion is what one's mind does instinctively.
He goes on to compare the square to whole notes in music. It's interesting when the feed reader gets lucky that way.
Me? I'm still getting used to 4:3 with my GRD2. I shot with it in 3:2 mode when I first got it, but I'm warming back up to 4:3. It's essentially the same proportion as my old peel-apart Polaroids and quarter-plate Speed Graphic.