Logged In: YES 
user_id=127598

Indeed. It was a permissions problem. I had trouble
controlling (and knowing) what user sendmail ran as, and
getting the group settings right. 

For the record, it seems that the lock file stays in place
even though the PID listed inside may no longer be running,
and the lockfile gets rewritten by the new process. I had
been trying to make it possible for the new process to
create a new file in the directory, which was the wrong goal.

Finally- isn't this lockfile policy a little screwy? PIDs
get recycled, so it would seem that if a stray process
coincidentally got the PID in the lockfile, roundup might
stall. Hopefully there's a flaw in this theory.

Thanks for all your help Richard!