This is the only bit of advice in your post that I completely disagree with. Three20 may have some nice UI bits, but it is a very poorly designed framework. I say this having looked at it before and after Facebook made a big redesign of it (breaking backwards compatibility and minimally documenting the changes.)

You can't add one view without adding a dozen other classes. Its not very well documented. It adds a large amount of bloat to the binary and drastically increases compile time.. I can go on, if you like.