Naturally, by running
FreeBSD you put yourself in the class of people who 1.
got root, and 2. Never lose it, but
just in case, here's what you can do:
Get the machine to write to hard disk, eg. with a
$cat /dev/urandom > deleteme.tmp
Wait for 1 second
Power cycle the box
Hurrah! Single user mode!
Note that this procedure is inadvisable, but then so is losing your password
ps. works with fbsd3.4, not tested with other releases.