I'd get to the login prompt, enter my credentials, see the motd, and be kicked out as zsh crashed, due to the change from 32-bit time_t to 64-bit time_t change. I'd also taken the security precaution of locking the root account.
If fixed this as follows:
- Reboot into single-user mode (boot -s at the boot> prompt)
- Mounted my filesystems (they all needed fsck running on them, before mount -a)
- Changed my login shell to /bin/sh (chsh -s /bin/sh <<username>>)
After that, it was a simple question of logging in and doing the usual; update my PKG_PATH to point at a 5.4 mirror, and running "pkg_add -u" to upgrade all my affected packages.
I then continued on to upgrade my system to OpenBSD 5.5.
A quick warning: This is one particular failure mode arising from not reading the docs. It may get you back into your system, but it's unlikely to fix all your woes if you don't read the docs.
So, the moral of the story is: ALWAYS READ THE DOCS.