You can now type accented character variants by pressing and holding on some letters on the keyboard. You get a pop-up menu with additional character choices. For example, press-and-hold on E and you get to choose from E, È, É, Ê, Ë, and Ę. The same trick also works for the ? and ! keys, allowing you to type ¿ and ¡.
Bonjour networking now works. E.g. if you’re using Mac OS X’s “Web Sharing” feature to serve a web site using Apache from your Mac, you can now access that server from your iPhone over Wi-Fi using your-machine-name.local, rather than having to use the IP address. This was a curious omission from iPhone 1.0 — as James Duncan Davidson wrote back in early July, after years of using Bonjour (née Rendezvous), “it just seems weird to use IP addresses again.” (Thanks to Scott P. Richert.)
Text message notification sounds: Settings → Sounds → New Text Message has a list of new alert sounds to choose from when you get a text message. They’re great sounds — iPhone has way better sounds than Mac OS X.
If you spot anything else, let me know and I’ll add to the list.
Update: The accented character trick and Bonjour networking both work on the iPod Touch as well.