According to Apple, the iPhone 4 has a new and better screen resolution:
3.5-inch (diagonal) widescreen Multi-Touch display 960-by-640-pixel res
For people who are coming to this thread looking for a solution to a mobile web interface, check out this post on the Webkit blog: http://webkit.org/blog/55/high-dpi-web-sites/
It seems that Webkit has solved this problem four years ago.