You can still bring up the menu on the Palm Pre while in landscape mode.  You may have noticed that when you put the Palm Pre into landscape mode in the browser or in email (after applying the Palm Pre Landscape mode email trick), the menubar on the top of the Pre disappears.  Good news: it's still there, you just can't see it.

Rdaex in our forums was amongst several PreCentral.net members to discover the trick. Just hit one of the volume buttons and the Pre will not only show the volume indicator at right, but at left the menubar will re-appear and you can tap the Menu to bring it up.