Followed these instructions: http://www.mysqlperformanceblog.com/2010/03/14/percona-xtradb-version-9-1/
------------------------------------
[artem@www tmp]$ sudo yum install Percona-XtraDB-server Percona-XtraDB-client
Loading "fastestmirror" plugin
Loading mirror speeds from cached hostfile
* epel: mirrors.tummy.com
* updates: mirrors.netdna.com
* base: mirrors.tummy.com
* addons: mirrors.netdna.com
* percona: repo.percona.com
* extras: mirror.raystedman.net
Setting up Install Process
Parsing package install arguments
Resolving Dependencies
--> Running transaction check
---> Package Percona-XtraDB-client-51.x86_64 0:5.1.45-10.rhel5 set to be updated
--> Processing Dependency: libmysqlclient_r.so.16(libmysqlclient_16)(64bit) for package: Percona-XtraDB-client-51
--> Processing Dependency: libmysqlclient.so.16(libmysqlclient_16)(64bit) for package: Percona-XtraDB-client-51
--> Processing Dependency: libmysqlclient.so.16()(64bit) for package: Percona-XtraDB-client-51
--> Processing Dependency: libmysqlclient_r.so.16()(64bit) for package: Percona-XtraDB-client-51
---> Package Percona-XtraDB-server-51.x86_64 0:5.1.45-10.rhel5 set to be updated
--> Running transaction check
---> Package MySQL-shared-compat.x86_64 0:5.1.40-2 set to be updated
--> Processing Conflict: mysql conflicts MySQL
--> Processing Conflict: mysql conflicts MySQL
--> Processing Conflict: mysql-server conflicts MySQL-server
--> Finished Dependency Resolution
Error: mysql conflicts with MySQL
Error: mysql-server conflicts with MySQL-server
-------------------------------
Artem,
Could you please show the output of "rpm -qa | grep -i -e mysql -e percona" ?