iPhone OS 4.0, which will ship with the next version of Apple's iPhone (and, later, the iPad), has been available to developers in beta form for a while now, in three consecutive versions. The newest, iPhone 4.0 Beta 4, just came out today, and it's got some new features we haven't seen before, as discovered by Gizmodo.
First is aesthetic—utilities like the calculator and compass are by default grouped into a "Utilities" folder on the homescreen, and there are a bunch of new wallpapers. Those wallpapers are of the traditional "fancy photography, landscape, simple pattern, and wood/stone/tile" variety, and they look fine enough, even if wallpaper on an iPhone homescreen still looks kind of weird.
Tethering, however, is the big deal here, showing up in the Network Settings area. Tethering is when a device with a 3G modem (like the iPhone) is attached (either wired or wirelessly via BlueTooth) to a computer, thus providing that formerly 3G-less device with 3G. Basically, plug your phone into your computer, and have 3G access on your computer wherever you go. WebOS does it through Wi-Fi hotspots, as do a few Android phones like the HTC Evo 4G, and all Android, BlackBerry, and Windows Mobile phones can tether with an app.
It appears that the iPhone's native tethering (it could always tether through a jailbreak app and, for a while, even without jailbreaking, for what it's worth) will be regulated by AT&T, predictably. When tapped, the option tells the user to contact AT&T to set up tethering—in all likelihood it'll cost extra. The HTC Evo 4G's tethering costs $30 a month extra on Sprint, to give an idea of what this might cost.
This is still a beta, and the feature may or may not stick around, but it's a good indication that tethering will be a part of iPhone 4.0. Let's just hope it's priced affordably.