5.0-current kernel (May 4) devfs not picking up slices from 4-STABLE disk

Hi,

Just started following the instructions in /usr/src/UPDATING to
go from 4-STABLE to 5-CURRENT and got stuck when booting the
new kernel (a GENERIC kernel). Note that as per instructions,
userland is still 4-STABLE at this point.

This GENERIC kernel is from about 2300GMT May 3, so fairly up-to-date
with an ATA disk.

When booting, it claims

'Root mount failed: 22'

should be '/dev/ad0s1a' (as in /etc/fstab)

using '/dev/ad0s1' succeeds for single-user mode,
but examination of /dev (i.e. devfs) indicates
no partition devices are being created in the /dev area
for ad0s1 (i.e. no ad0s1a, ad0s1b, etc..)

The only recent conversation (in -current) about this sort of thing
indicated an ata (ata_tag=1) problem, but mine is set to '0' anyway.

any ideas or suggestions for fixing this out there?

Cheers,
Mark