From the MyFonts Web Site:
WhatTheFont for iPhone connects directly to MyFonts’ acclaimed font identification service, which has been helping customers pinpoint mystery fonts for 10 years. It works via Wi-Fi or the mobile phone network, so you can get your font fix right there on the spot.
For iPod Touch Users:
You may not have a camera, but you can still get in on the action! WhatTheFont for iPhone will identify fonts in images saved from apps such as Safari and Mail.
The WhatTheFont app is free to download from the iTunes App Store
WhatTheFont is also available (free) as a web database.