Switching Logos. This is shown in the picture I use for this post. You can grab the AT&T logos (curiously enough, the Apple engineers also included logos for Cingular, T-Mobile, and Vodafone). Rundown of the hack: copy the Default_CARRIER and FSO_CARRIER files to your computer, use fixpng to convert them to standard PNG format, use Photoshop to edit the text, and store the altered versions back on the iPhone (using standard PNG seems to work fine). Reboot and enjoy.
Adding ringtones. Place your .m4a files into /Library/Ringtones. I added a couple of sound effects files from a royalty-free CD I had around and they worked fine.
Update widget icons. If you feel like hacking, go into any of the widget applications in /Applications, grab a logo and edit it. These seem to use standard PNG files and don't need the fixpng extra step.
Reorder your widget icons. The DisplayOrder.plist file in /System/Library/CoreServices/Springboard.app specifies the order in which your icons display. Grab the file, use plutil to convert the file to standard text (plutil -convert xml1 DisplayOrder.plist), edit the widget order, and store it back on the iPhone. Then power off and back on.
Reveal the two "secret apps". The iPhone has four extra widgets listed in the "Special" section of DisplayOrder.plist. If you copy these to the main ("icon list") portion, two of them appear as interactive widgets. They are: Demo App and Field Test. Field test simply takes you to the same field test screen as dialing *3001#12345#* does. Demo app doesn't appear to do anything on my iPhone but looking at the strings inside its executable reveals that it's meant to show movies. My guess? It's for displaying iPhones at the store. I haven't yet tried adding a movie named Demo.
Thanks to all the #iphone folks, especially brutalentropy