One of my favorite Macworld columnists, Dan Frakes, does a review of iPhone wide screen email editors and gives Wide Email from Lizzardwerks the highest rating.
http://www.macworld.com/article/136580/2008/11/wideformatemail.html
For only a $1 you not only get to compose messages in landscape mode but you also get productivity boosters like text macros (ala textexpander on the Mac) and the ability to compose email templates.
