Boot fails after fresh install of 3.3

[Greg Black <gjb_(_at_)_gbch_(_dot_)_net>]

I've just done my first OpenBSD install (on a laptop, via FTP on
my LAN).  The boot from floppy went fine; all the steps of the
install appeared to go fine and eventually I was invited to give
the halt command and let the machine reboot.

The boot process showed:

    reading boot....
    Bad magic
    _

And that was it.

The only potentially odd thing I can think of is that this
laptop has three operating systems on its 20 G disk: Windows-ME
in the 1st slice; FreeBSD-4.3 in the second slice; and (until
today) FreeBSD-5.0 on the third slice.  I wanted OpenBSD to
replace FreeBSD-5.0 on that third slice and expected it to go
smoothly.

I used the 3.3 i386 snapshot stuff from the OpenBSD ftp site.

Have I struck a known problem?  Or is there some obvious step
that I have missed?  I have read all the documentation that I
could find, but did not discover a reference to this issue.

Any guidance will be most welcome.

Greg