$> sqlplus / as sysdba
sqlplus: error while loading shared libraries: /u01/app/oracle/product/11.2.0/dbhome_1/lib/libclntsh.so.11.1: cannot restore segment prot after reloc: Permission denied
The quick way to get Oracle working is to use setenforce:

$> setenforce 0
$> sqlplus / as sysdba
SQL*Plus: Release 11.2.0.1.0 Production on Wed Jan 13 15:31:35 2010
Copyright (c) 1982, 2009, Oracle. All rights reserved.
…
However, this solution will only work until you reboot the server.
To permanently fix this issue you need to modify the SELinux configuration file:

$> vi /etc/selinux/config
Change:
SELINUX=enforcing
To:
SELINUX=disabled
You should also be able to use permissive mode on single instances, if you change to this mode BEFORE you install Oracle:
SELINUX=permissive
CRS will not like permissive mode, so this does not apply for RAC installations.
Now SQL*Plus works:

$> sqlplus / as sysdba
SQL*Plus: Release 11.2.0.1.0 Production on Wed Jan 13 15:31:35 2010
Copyright (c) 1982, 2009, Oracle. All rights reserved.