This is resolved for ius-el5, and ius-el4 stable... please note that it will take up to 24 hours for all mirrors to sync with dl.iuscommunity.org:

[root@centos5-2 ~]# yum clean all
Cleaning up Everything

[root@centos5-2 ~]# yum install mysql51 mysql51-server
epel                                                                         | 3.4 kB     00:00     
aaad8b1e3323490e8d92b6c5cdea04f96263749b-primary.sqlite.bz2                  | 2.7 MB     00:00     
base                                                                         | 2.1 kB     00:00     
primary.sqlite.bz2                                                           | 2.0 MB     00:00     
updates                                                                      | 1.9 kB     00:00     
primary.sqlite.bz2                                                           | 320 kB     00:00     
addons                                                                       |  951 B     00:00     
primary.xml.gz                                                               |  203 B     00:00     
ius                                                                          | 1.9 kB     00:00     
primary.sqlite.bz2                                                           |  91 kB     00:00     
extras                                                                       | 1.1 kB     00:00     
primary.xml.gz                                                               | 117 kB     00:00     
extras                                                         290/290
Setting up Install Process
Parsing package install arguments
Resolving Dependencies
--> Running transaction check
---> Package mysql51.x86_64 0:5.1.41-1.ius.el5 set to be updated
--> Processing Dependency: mysqlclient15-devel for package: mysql51
--> Processing Dependency: mysqlclient15 for package: mysql51
---> Package mysql51-server.x86_64 0:5.1.41-1.ius.el5 set to be updated
--> Running transaction check
---> Package mysqlclient15-devel.x86_64 0:5.0.85-1.ius.el5 set to be updated
---> Package mysqlclient15.x86_64 0:5.0.85-1.ius.el5 set to be updated
--> Finished Dependency Resolution

Dependencies Resolved

====================================================================================================
 Package                        Arch              Version                      Repository      Size
====================================================================================================
Installing:
 mysql51                        x86_64            5.1.41-1.ius.el5             ius            5.5 M
 mysql51-server                 x86_64            5.1.41-1.ius.el5             ius             11 M
Installing for dependencies:
 mysqlclient15                  x86_64            5.0.85-1.ius.el5             ius            1.7 M
 mysqlclient15-devel            x86_64            5.0.85-1.ius.el5             ius            187 k

Transaction Summary
====================================================================================================
Install      4 Package(s)         
Update       0 Package(s)         
Remove       0 Package(s)         

Total download size: 18 M
Is this ok [y/N]: y
Downloading Packages:
(1/4): mysqlclient15-devel-5.0.85-1.ius.el5.x86_64.rpm                       | 187 kB     00:00     
(2/4): mysqlclient15-5.0.85-1.ius.el5.x86_64.rpm                             | 1.7 MB     00:00     
(3/4): mysql51-5.1.41-1.ius.el5.x86_64.rpm                                   | 5.5 MB     00:00     
(4/4): mysql51-server-5.1.41-1.ius.el5.x86_64.rpm                            |  11 MB     00:01     
----------------------------------------------------------------------------------------------------
Total                                                               9.3 MB/s |  18 MB     00:01     
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing     : mysqlclient15                                     [1/4] 
  Installing     : mysqlclient15-devel                               [2/4] 
  Installing     : mysql51                                           [3/4] 
  Installing     : mysql51-server                                    [4/4] 

Installed: mysql51.x86_64 0:5.1.41-1.ius.el5 mysql51-server.x86_64 0:5.1.41-1.ius.el5
Dependency Installed: mysqlclient15.x86_64 0:5.0.85-1.ius.el5 mysqlclient15-devel.x86_64 0:5.0.85-1.ius.el5
Complete!

[root@centos5-2 ~]# mysql -V
mysql  Ver 14.14 Distrib 5.1.41, for redhat-linux-gnu (x86_64) using readline 5.1