Yum errors

[dan]

    I've been using yum all morning with no problems searching for and 
listing packages. Then I try it again and get the following error:

---------------
[dan@loki dan]$ yum list libart
error: cannot get shared lock on /var/lib/rpm/Packages
error: cannot open Packages index using db3 - Operation not permitted (1)
error: cannot open Packages database in /var/lib/rpm
Traceback (most recent call last):
  File "/usr/bin/yum", line 30, in ?
    yummain.main(sys.argv[1:])
  File "/usr/share/yum/yummain.py", line 152, in main
    (log, errorlog, filelog, conf, cmds) = parseCmdArgs(args)
  File "/usr/share/yum/yummain.py", line 71, in parseCmdArgs
    conf=yumconf(configfile=yumconffile)
  File "/usr/share/yum/config.py", line 130, in __init__
    self.yumvar['releasever'] = self._getsysver()
  File "/usr/share/yum/config.py", line 255, in _getsysver
    idx = ts.dbMatch('provides', self.distroverpkg)
TypeError: rpmdb open failed
[dan@loki dan]$
----------------------

I haven't installed anything or changed anything with yum.conf. If I try 
to do anything with rpm I get the same error. Trying it with sudo 
doesn't work either.

Any ideas?

Thanks,
Dan