Apple released the first update for the iPhone this afternoon. You can access it via iTunes. Plug in your iPhone. On the iPhone main tab, check for updates. The update takes 5 minutes or so. I've installed the update and it went smoothly.
The best information so far besides a published list of security bug fixes from Apple is just anecdotal at this point in the comments at Engadget.
But some bug fixes were reported in the comments for YouTube, iPod crashes while using Safari, a lot more stability in Safari when using Ajax pages, being able to see folders in IMAP accounts, fixing a bug in Safari where you couldn't close pages under certain circumstances, fixing a bug in Google Maps. I'm sure it won't take long for someone to compile a thorough list of fixes.
I, of course, hoped the update would add more functionality, and there is a tiny bit, but this is a good v1 upgrade within 30 days, so I am content for now. I read a note from Blackfriars saying a bigger upgrade was in the works. Here's hoping it comes in August. I want Apple to take their time and get stuff right, and I am eager for more.