About a month ago, Google decided to release a new feature that allowed using the arrow keys to navigate up and down in the thread list. Unfortunately, that meant that the arrow keys no longer work for scrolling the message body when the Preview Pane lab is enabled. I finally had enough, and wrote a Greasemonkey script to re-enable scrolling the preview pane with arrows. I’ve done some testing and it seems pretty stable. If you’re interested in trying it out, the link is below. If you have any feedback, you can leave it in the forum on the userscripts site.

Gmail Preview Pane Arrow Scrolling