Aaron emailed me a screenshot of Mail.app taken from an episode of the TV show Weeds (Series 2, Episode 5), which you can see below. Another screenshot has been posted on Flickr. It seems to show a message preview pane in a drawer on the right.
I am suspicious. It looks like a photoshop job to me.
Tiger icons and Panther Mail drawers seem odd. The Apple menu seems to be missing, the app is identified in the menubar as “Email” rather than “Mail”, the Applescript menu is where it was positioned in Panther, whereas it is on the right in Tiger Mail. The Junk mailbox appears below Trash in Tiger Mail but these are reversed in the screenshot (and these cannot be reordered):

On the other hand, it appears to be a functioning app. I’ve not seen the episode, but Aaron tells me that “the cap comes from a longer sequence that shows the character browsing through an inbox full of mail, with multiple messages popping up in that pane.”
A puzzle. What do you think?
[Thanks to an Apple Mail demi-god for technical advice. You know who you are!]