[root@host]# df -h
df: cannot read table of mounted file systems
[root@host]# cat etc/mtab
Parse error at byte 6 (line 1): unexpected token
This is often related to a disk space issue. A post from insanelabs.com recommended freeing up space and then rebuilding /etc/mtab from /proc/mounts like so:
[root@host]# grep -v rootfs /proc/mounts > /etc/mtab
Seemed to do the trick, df is now returning expected results.