It's been something like ten years since the last time I did 
anything involving a user crontab (as opposed to the system
crontab, /etc/crontab).  I ran, as me, crontab(1) and created
this crontab:

xela:~# crontab -l
55 9 * * 6    /home/xela/bin/crontest

The script in question (appended below) is very straightforward.
It's a stripped down version of the much longer script I really
want to run from cron, but it does the main things that script
does:  write to a file in the user's homedir, and write to a file
in /tmp, then mail that file.

cron seems to just ignore it entirely; the only thing in its log
is me editing; series of LIST, BEGIN EDIT, REPLACE, and END EDIT
entries corresponding to each time I've edited the crontab.  And
that's it.

There's something that looks deceptively like a clue in the
cron man page:

       If the allow file exists, then you must be listed therein in  order  to
       be  allowed  to use this command.  If the allow file does not exist but
       the deny file does exist, then you must not be listed in the deny  file
       in  order  to use this command.  If neither of these files exists, then
       depending on site-dependent configuration parameters, only  the  super
       user  will be allowed to use this command, or all users will be able to
       use this command.

Inially the machine had no allow or deny file.  The fact that I
could run crontab as xela suggested there was no "site-dependent
configuration parameter" issue.  But man pages are often so
poorly written that I can easily imagine that "allowed to use
this command" might really mean "allowed to run jobs out of their
crontab".  So I went hunting for some sign of that "site-dependent
configuration", without any hints from the man page as to where it
might be hidden.  I found nothing in /etc, nothing in /var/cron, 
and then gave up and created a /var/cron/allow with me in it.  Ran
crontab -e, changed the time to about ten minutes in the future,
waited to see if it ran.  And when it didn't, started this mail.

Any and all clues greatly appreciated.

---Alex

Here's the stripped-down script I'm asking it to run (slightly
edited for posting here:  see $DOMAIN).  Note that I have also
tried a varient where $HOME and $USER were hard-coded, in case
cron's environment is more cripled than I think.)

#!/bin/sh

f_start-message() {
    MSGFILE="/tmp/crontest"
    rm -f $MSGFILE
    echo "To: ${DEST}@${DOMAIN}" > $MSGFILE
}

DOMAIN="example.com" 
DEST="$USER"
DATESTAMP=`date +'%Y%m%d_%H%M%S'`

# test writing to a file
echo "$0" > ${HOME}/tmp/$DATESTAMP

# test sending mail
f_start-message
echo "Subject:  $0 ran" >> $MSGFILE
echo "" >> $MSGFILE
echo "$DATESTAMP" >> $MSGFILE
/usr/sbin/sendmail -t -i < $MSGFILE