I recently downloaded the iPhone OS 4.0 beta 3 which is only available to iPhone Developers.
Some of the new features include:
The Home screen has a Mac OS X style dock, and most of the system default icons have a new icon.
You can have "folders"to store icons inside of.
You can finally have a home screen wallpaper of your choice.
It also features multitasking with a built in task manager
Holding down on a icon in the task manager will bring these red circles with white dash up. Click on them to close a background app.
Photos are now organized a lot like iPhoto on the Mac. You can even have Albums, Events, Faces, and Places!
I am looking forward to the new features that 4.0 will have to offer!