I had perhaps a similar experience, which leads me to think the
-current upgrade bootblock procedure isn't quite right.

On a vanilla PII-400 box running 1.6.2 with root on wd0, I upgraded to
-current of early february with a CD I built myself from sources (with
a custom kernel instead of GENERIC set to be installed by sysinst).  I
had never had a boot selector, and the machine immediately reset on
booting.

I started to boot from cd, but stopped it and did

boot hd0a:netbsd

(or wd0a - I don't remember).  From this I could see that the system
was fine from /netbsd and userland.  I then ran installboot, I'm
pretty sure to the raw device and it has been fine.

I did a fresh install with my custom CD onto a new box and that worked
with zero problems.

-- 
        Greg Troxel <gdt@ir.bbn.com>