I can attest to the fact that crontabs work under the alpha-6 load, as well as some other things that are broken under the load that comes with the Plug.
If you want to continue diagnosing your problem, I'd suggest looking at /var/spool/cron/crontabs/: Does this directory exist? Are the permission and owner/group correctly set? Here's what it should look like:
# ls -ld /var/spool/cron /var/spool/cron/crontabs
drwxr-xr-x 5 root root 4096 Jul 10 21:06 /var/spool/cron
drwx-wx--T 2 root crontab 4096 Nov 16 2008 /var/spool/cron/crontabs
(I recall that there was a section of /var that was overlayed by a ramdisk, and needed to be recreated on every boot in your load. This might be the problem.)