With the iPhone's original 1.x software, when you send a photo, web link, etc., you could only send it from the default account. With version 2.0, however, you can now send it from any account on the iPhone.
After you create a message to send the photo, link, etc., notice that there is a header line labeled 'CC/Bcc, From.' Tap this line, and it will expand into three lines: Cc, Bcc, and From. Tap the From line, and a menu will appear to allow you to select any account you have on the iPhone.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080805095930468