From a feature standpoint Apple has created with the iPod touch 4G an almost exact copy of the iPhone 4. You get the retina display, two cameras for FaceTime calling and more. What is missing is the 3G connectivity and GPS. Now that the first iPod touch 4G have shipped and people take it apart more differences to the iPhone 4 surface. The new iPod touch is rather thin compared to the iPhone 4. The downside of this is that the iPod touch has a battery with 3.44Whrs compared to 5.25Whr of the iPhone 4 according to images on 9 to 5 Mac.
In the iPod touch 4G tear down of iFixit it got unveiled that the iPod touch has only 256MB RAM instead of the 512MB that can be found on the iPhone 4. Less RAM could mean less multi-tasking and app performance and of course for Apple it means less cost.