Boot problems going from 5.2-R to -CURRENT

I've just set up a new system, using 5.2-R as my starting point.  Right
after installing, I supped to -CURRENT, rebuilt, and rebooted.  I was
running headless, and the box didn't come back up properly after about 5
minutes, so I hit reset.  Next boot, same problem, so I moved a monitor
over, and it had hung after trying to mount root (from /dev/ad0s1a).

(cvsup time was early this afternoon.)

The system was pretty much frozen -- num/caps/scroll lock responded, but
that's it.  So I hit reboot again, and tried to boot into single user mode.
Then I got this:

    WARNING: R/W mount of / is denied.  Filesystem is not clean - run fsck.
    WARNING: /usr was not properly dismounted
    WARNING: /var was not properly dismounted
    WARNING: R/W mount of / is denied.  Filesystem is not clean - run fsck.
    WARNING: /usr was not properly dismounted
    WARNING: /var was not properly dismounted
    .
    .
    .

And so on.  They just kept on coming.  So I hit reset, and booted into 'Safe
Mode'.  I was successfully able to bring up the system, and dropped down to
single-user mode, unmounted /usr, and fsck'ed it.  But I couldn't unmount
/var, and I've never tried re-mounting a partition ro under FreeBSD, so that
didn't go over well (but that's where my starting point will be for bringing
the system back up).

Anyhow, next reboot, try it normally.  Upon mounting /, I get the same
behaviour as I did initially.  This time, two lines were spit out about
fsck_ufs:

    pid 86 (fsck_ufs), uid0: exited on signal 8
    pid 87 (fsck_ufs), uid0: exited on signal 8

And I'm now getting the repetitive lines about R/W mount of / being
denied.  I remember seeing a commit about opening a device R/W even if R/O
access is requested, which feels like it might be involved...?

This is a pretty straight VIA C3 CPU, on a DFI motherboard sporting a VIA
chipset.  I'd never previously had any ATA issues, and system had booted
into 5.2-R a number of times without issue.  No fancy hardware -- 40GB
Samsung ATA100 drive, fxp0, and that's about it.