You can get google maps for the Iphone, but no Voice guidance so far.
MotionX GPS Drive is darn good, but the voice guidance part will cost you $20 a year. That's less than $2 a month, well worth it IMHO. The only thing missing from it that Google maps has is voice recognition. If you never used voice recognition before then you won't miss Google maps after trying MotionX Drive. It actually does a lot more than Google maps once you get into it.