#import "Three20/Three20.h"
Fails. Go download it and build a sample app. If that works, then make a new app from the templates provided, then watch as it fails spectacularly to find the right HEADER_SEARCH_PATH.
1) I've gone to the info page for the TARGET and not the PROJECT (Well, I tried both) and tried a bunch of different paths for the HEADER_SEARCH_PATH field.
2) I've tried dragging and dropping the dependent files from the finder and COPYING or just referencing both fails.