Comment 9 for bug 1084309

A possible solution

yum install Percona-XtraDB-Cluster-shared
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: centos.mirror.linuxwerk.com
 * epel: vesta.informatik.rwth-aachen.de
 * extras: centos.mirror.linuxwerk.com
 * updates: ftp.hosteurope.de
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package Percona-XtraDB-Cluster-shared.x86_64 1:5.5.31-23.7.5.438.rhel6 will be obsoleting
---> Package mysql-libs.x86_64 0:5.1.69-1.el6_4 will be obsoleted
--> Processing Dependency: libmysqlclient.so.16()(64bit) for package: 2:postfix-2.6.6-2.2.el6_1.x86_64
--> Processing Dependency: libmysqlclient.so.16(libmysqlclient_16)(64bit) for package: 2:postfix-2.6.6-2.2.el6_1.x86_64
--> Running transaction check
---> Package Percona-Server-shared-compat.x86_64 0:5.5.32-rel31.0.549.rhel6 will be obsoleting
--> Finished Dependency Resolution

Dependencies Resolved

=========================================================================================================================================
 Package Arch Version Repository Size
=========================================================================================================================================
Installing:
 Percona-Server-shared-compat x86_64 5.5.32-rel31.0.549.rhel6 percona 3.4 M
     replacing mysql-libs.x86_64 5.1.69-1.el6_4
 Percona-XtraDB-Cluster-shared x86_64 1:5.5.31-23.7.5.438.rhel6 percona 648 k
     replacing mysql-libs.x86_64 5.1.69-1.el6_4

Transaction Summary
=========================================================================================================================================
Install 2 Package(s)

Total download size: 4.1 M
Is this ok [y/N]: n

A yum upgrade now will be happy, too.

AFAIU, the problem is, that the Percona repo conflicts with mysql-libs from base.
You are conflicting with the CentOS Base repository here!
Not sure if this conflict could cause trouble during future installs or upgrades.