I haven't seen any iPhone app that I would consider killer but this is pretty damn cool.
"Your world just got smaller thanks to a speech-to-speech translation app for your iPhone. You talk in English-it talks back in Spanish. You talk in Spanish-it talks back in English. Simple as that. No data charges requiredjust your voice."
I have downloaded it and tried it out. It works quite well for everyday speech but fails badly if you veer off...