Google offers Google Translate now with an optimized interface for the Apple iPhone.
Google Translate for iPhone is optimized for speed, supports all of the existing Google Translate language pairs, and uses a client-side data-store on your iPhone to hang on to your past translations so you always have them at hand, even if you can't use the local data network.
To try Google Translate for iPhone, point your iPhone or iPod Touch web browser to www.google.com and choose the "more" tab. Or you can go directly to translate.google.com in your browser. More details on Google Mobile Blog.

Google Translate is one of the most valuable services to me from the Search Giant. It works well enough even for Chinese or Korean. Despite Google pursuing their own Mobile Phone OS with Android they are very much in love with the iPhone. In July Google announced the Google Mobile App for the iPhone for instance.