I ran grub-install /dev/sda and (just to be sure) rebuilt the initramfs. After rebooting, kernel 2.6.33-2 loaded successfully.
Thank you for the help. I don't know why everything wasn't updated automatically; perhaps grub-pc 1.98 didn't re-install itself during the upgrade. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org