So I tried a new CD. Same thing. This time I was using tty2 when it stopped. I also ran ubiquity in debug mode in hopes of finding anything.
Found a bunch of segfault alerts in the syslog and dmesg; ubiquity, Xorg, gzip, debconf, among others. Top, running on tty2, froze too, but the other consoles still worked.
Last message in /var/log/installer/debug was something like debconf 1 Configuring keyboard... -> 0 OK.
It's just not meant to be...