What am I doing wrong?
Titanium.Platform returns the same results for either 3gs or iPhone 4 in the simulator. For example, both the 3Gs and 4 return the same screen resolution of 320x480 in the simulator. This is true in the Kitchen Sink app also.
You can change the iPhone 4 size in the top menu under Window/Scale
Try detecting the 'density' from the DisplayCaps object :