Turns out, prior to rebooting, I had installed APE, which others had found causes this issue.
The solution was given by apple's troubleshooting pages:
Mac OS X 10.5: "Blue screen" appears after installing Leopard and restarting
In case the link dies, here is the portion that I used:
Issue or symptom
After completing an upgrade installation of Leopard and restarting the computer, a "blue screen" may appear for an extended period of time.
- Mac OS X 10.5 Leopard
You may have third-party "enhancement" software installed that does not work with Mac OS X 10.5 Leopard. Use one of these solutions.
Option A: Use another Mac to remove application enhancement software
If you have a second Mac that has a FireWire port (even if it's not running Leopard), and a FireWire cable that you can connect to both computers, use this solution:
- Start your original, affected computer in Target Disk Mode by holding the T key, and connect it to your second computer. Your affected computer's hard disk volume(s) should appear on the desktop of the second computer.
- Open the second computer's mounted disk volume (where Leopard is installed), then open the System folder, then the Library folder, and finally the SystemConfiguration folder.
- Locate "ApplicationEnhancer.bundle" and move it to the Trash. Important: Do not delete any other files from your Leopard disk.
- Enter an admin account name and password if prompted.
- Eject the mounted volume(s). (Disconnect the FireWire cable if you want).
- Turn off the original computer and start it up again (holding no keys).
If the issue persists, or if the "ApplicationEnhancer.bundle" file was not on your Leopard volume, use Option B.
Option B: Reinstall Leopard
If you can't use option A, it may be necessary to perform an Archive and Install installation of Leopard. Archive and Install moves your existing Mac OS X system files to a folder named Previous System, and then installs a new copy of Mac OS X on the selected volume. Mac OS X–installed applications, such as Address Book and Safari, are archived, and new versions are installed in the Applications folder. Applications, plug-ins, and other software may have to be reinstalled after an "Archive and Install." This is covered on page 7 of the Install & Setup Guide included on the Leopard DVD.
You will probably want to check "Preserve user and network settings" when starting the installation.
Note: After installation, verify each third-party software product is compatible with Leopard before reinstalling it, especially any application "enhancement" software.
Option C: Use the command line (advanced) to remove application enhancement software
Try this solution if you are comfortable using Terminal and have certain application enhancement installed. The software may be removed following the below steps:
- Start up in single-user mode by holding Command-S after restarting the computer. Note that single-user mode always uses the US English keyboard layout.
- Type these commands, each on a single line followed by Return:
/sbin/mount -uw /
rm -rf /System/Library/SystemConfiguration/ApplicationEnhancer.bundle
- If the issue persists, use option A or B above.