The default home screen is different. Instead of all your apps being spread out when you first turn on your iPhone, now some—the clock, calculator, compass, and voice memo apps—are already sitting in a folder called "Utilities.'
Oh, and speaking of home screens, there are now finally some new default wallpapers that aren't busy or distracting and could actually serve as decent home screen wallpapers.
Aside from those new wallpapers and an option to toggle off the ability to send group messages, there don't appear to be any major differences in the settings for this beta...except for tethering!
There's a tiny change to the Photo app in that you can finally view your Photo Roll in a different orientation—unless you have Orientation Lock turned on that is. [Thanks to Habib for catching this!]
There are some very pleasant non-cosmetic changes since the last beta, too: There isn't an odd silent pause when you pick up a phone call now and the Maps app seems snappier and more precise than ever. Actually, just about all the apps somehow feel faster and more responsive.
Those are the changes from iPhone OS 4.0 Beta 3 that we've discovered so far. We'll update as we find any others and we invite you to send in screenshots and descriptions of what you come across.