The Stanford iTunes U Courses should not be overlooked. They do a great job of taking you from crawling to running in no time. I enjoyed doing the homework too, it really increased my learning experience.
Winter 2010- http://itunes.apple.com/us/itunes-u/iphone-application-devel...
Spring 2009-http://itunes.apple.com/us/itunes-u/iphone-application-progr...
Spring 2011 (Starting Soon)-http://www.stanford.edu/class/cs193p/cgi-bin/drupal/