Mail.app in Leopard creates an IMAP folder for its To Do list and by default calls it "Apple Mail To Do." It's hidden when looking at the IMAP box in Mail, but it's not hidden on the iPhone. I would rather not have it at the top of the list on the iPhone, so changed its name.
To change the default name, quit Mail. Open com.apple.mail.plist in your favorite editor. Look for the Mail Accounts section; each account has its own list within that section. Find the ToDosMailboxName entry, and change the associated value to your preferred title and save. Important: Before you launch Mail.app, you need to log into your IMAP account with a web interface (or command line if you know how), and change the Apple Mail To Do folder to its new name.
[robg adds: Apple's Mail prefs are stored in binary form, so you'll need to use a binary-capable editor such as Plist Edit Pro, BBEdit, etc., or convert the plist to XML first as described in this hint.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080327194950551