Google has released an optimized version of Google Translate for iPhone users. This lets any one to translate from one language to other, same as the web version.
Google translate on iPhone also remembers your past translations using a client-side data storage on your iPhone, so that you can access them at any time you want. The tool is optimized to use minimal bandwidth but make sure you check your data limits particularly when you are on roaming.
Many commentators have noted on the Google Mobile blog that this version strips down useful site translation feature. There is no way to get the site translation with this version as of now.