Thank you everyone so much for your help... now I can go to bed!
I think it's fixed now... I'll check again tomorrow after some sleep but 
some is coming in and no more lock file errors in the logs...
the permissions were:
drwxrwxr-x   2 root    lp        4096 Aug 18 23:15 /var/spool/mail
-rws--x--x   2 root     bin        40960 Nov 15  1996 /usr/bin/binmail
-rwsr-xr-x   4 root     bin       204800 Feb  2  1997 /usr/sbin/sendmail
so I did a chmod 1777  and chgrp mail  to /var/spool/mail...
Still saw nothing coming in... stopped and restarted sendmail and it 
started getting mail.
Still not sure what caused the sudden change...
Thanks again!     time for some ZZZzzz's
 >Can someone please help me figure out why there's no incoming mail?
 >
 >The machine responds from externally, web server is running, I can send
 >mail out...
 >
 >In the messages, for each incoming mail I get the following message:
 >Aug 18 22:07:03 suhep binmail[722]: Cannot create mail lock file
 >/usr/spool/mail/USER.lock
 >
 >I also see some like:
 >Aug 18 22:10:00 suhep sendmail[929]: AA00927: to=<USER_at_suhep.phy.syr.edu>,
 >delay=00:00:01, stat=Can't create output
 >
 >So the machine responds and is reachable in and out, and no disks are full.
 >
 >
 >
 >Please reply to dkirkpa1_at_twcny.rr.com since I can't get any mail here!
 >
 >- Dan Kirkpatrick - dkirk_at_physics.syr.edu -
 >|         Systems Administrator           |
 >|          Physics Department             |
 >|    Syracuse University, Syracuse, NY    |
 >-------------------------------------------
Received on Thu Aug 19 1999 - 04:14:23 NZST