The first time you execute up2date you need to register with the Red Hat network. However, it would accept my username and password. It would keep sending me back to the login screen or provide me with the option to register later. (Which means you can't download updates until you register.)
I noticed on the command line from which I launched up2date:
[root@server ~]# rhn_register
An error has occurred:
See /var/log/up2date for more information
The log file was of no use... Searching google and the Red Hat knowledge base didn't help either. So I tried to run up2date without the GUI using the --nox option and I saw this:
[root@server ~]# up2date --nox
There was an SSL error: [('SSL routines',
'SSL3_GET_SERVER_CERTIFICATE', 'certificate verify failed')] A common cause of
this error is the system time being incorrect. Verify that the time on this
system is correct.
Bingo! I checked the system date/time and the year was 2008... Once I fixed that up2date ran fine. I have no idea why it would care if the system date was in the past.