We have CentOS 5.5 production systems with a major Cloud VM vendor and I have replicated the results with CentOS 5.5 on local VM's (ISO install) with my own VM platform.
The remote database server is listening on eth1.
eth0 is our public routable IP and eth1 is our 10.x.x.x private subnet.
We do not expose the mysql servers on the public interfaces. All of our web applications access the database servers by IP on the 10.x.x.x subnet on eth1.
The apache server where phpmyadmin is installed also has other virtual hosts that have wordpress instances among other things, that use the mysqli interface to connect to the same remote database server and they all work fine.
The apache server also has a local mysql server that I can successfully connect phpmyadmin to using localhost or 127.0.0.1 for the db host name with cookie authentication.
The apache server where phpmyadmin is installed I can use the bash mysql client and connect to the remote database server with my credentials.
I have tested CentOS 5.5 with RPM installed httpd and RPM installed PHP 5.1 and PMA 2.x, and CentOS 5.5 RPM installed Apache, my compiled PHP 5.2.3 and PMA 3.x. Both with the same result.
Logins to any remote server over TCP with cookie auth fail with a "#1045 Cannot log in to the MySQL server" and tcpdump shows no packets on the interface to the DB server.
Connections to the local database server via PMA 3.x, my compiled php 5.2 over tcp using localhost, port 3306, cookie auth connects right away.
Connections to the local database server via PMA "mysqli://localhost:3306" shows no traffic in tcpdump on lo, eth0 or eth1 so I'm assuming it uses the local socket for this.
I have tested Ubuntu 10 with Apt installed php, apache, mysqladmin and it all works out of the box. I would love to just throw ubuntu up there but work won't allow a mixed linux environment. It HAS to be CentOS.

Just to make sure I am not crazy I made the test file below and connected to my remote database from the same virtualhost as phpmyadmin using the same username and password (not root). It works perfectly and I get the results from my mysql.user table. I have tested this php both from the CLI and through Apache.

<?php

$mysqli = new mysqli('10.x.x.x.', 'myuser', 'mypass', 'mysql');

$result = $mysqli->query(

'SELECT * from user');

while( $row = $result->fetch_assoc() ){

print_r($row);

echo "\n";

}

$result->close();

$mysqli->close();

And my PMA config files look like this

THIS DOES NOT WORK

<?php

* Generated configuration file

* Generated by: phpMyAdmin 3.3.5 setup script by Piotr Przybylski <piotrprz@gmail.com>

* Date: Thu, 26 Aug 2010 10:32:53 -0600

/* Servers configuration */

$i = 0;

/* Server: glacier [1] */

$i++;

$cfg['Servers'][$i]['auth_type'] = 'cookie';

$cfg['Servers'][$i]['host'] = '10.x.x.x';

$cfg['Servers'][$i]['extension'] = 'mysqli';

$cfg['Servers'][$i]['bookmarktable'] = 'pma_bookmark';

$cfg['Servers'][$i]['relation'] = 'pma_relation';

$cfg['Servers'][$i]['table_info'] = 'pma_table_info';

$cfg['Servers'][$i]['table_coords'] = 'pma_table_coords';

$cfg['Servers'][$i]['pdf_pages'] = 'pma_pdf_pages';

$cfg['Servers'][$i]['column_info'] = 'pma_column_info';

$cfg['Servers'][$i]['history'] = 'pma_history';

$cfg['Servers'][$i]['designer_coords'] = 'pma_designer_coords';

$cfg['Servers'][$i]['verbose'] = 'somename';

$cfg['Servers'][$i]['port'] = 3306;

$cfg['Servers'][$i]['socket'] = '';

$cfg['Servers'][$i]['connect_type'] = 'tcp';

$cfg['Servers'][$i]['user'] = '';

$cfg['Servers'][$i]['password'] = '';

$cfg['Servers'][$i]['AllowRoot'] = false;

$cfg['Servers'][$i]['AllowDeny'] = array (

'order' => 'allow,deny',

);

/* End of servers configuration */

$cfg['blowfish_secret'] = '4c74332d81b5c0.29678885';

$cfg['UploadDir'] = '';

$cfg['SaveDir'] = '';

$cfg['DefaultLang'] = 'en-utf-8';

$cfg['ServerDefault'] = 1;

$cfg['ForceSSL'] = true;

$cfg['AllowAnywhereRecoding'] = true;

$cfg['UseDbSearch'] = false;

$cfg['ShowPhpInfo'] = true;

$cfg['LeftDisplayServers'] = true;

$cfg['DisplayServersList'] = true;

THIS WORKS

<?php

* Generated configuration file

* Generated by: phpMyAdmin 3.3.5 setup script by Piotr Przybylski <piotrprz@gmail.com>

* Date: Thu, 26 Aug 2010 10:34:31 -0600

/* Servers configuration */

$i = 0;

/* Server: localhost [1] */

$i++;

$cfg['Servers'][$i]['verbose'] = 'localhost';

$cfg['Servers'][$i]['host'] = 'localhost';

$cfg['Servers'][$i]['port'] = 3306;

$cfg['Servers'][$i]['socket'] = '';

$cfg['Servers'][$i]['connect_type'] = 'tcp';

$cfg['Servers'][$i]['extension'] = 'mysqli';

$cfg['Servers'][$i]['auth_type'] = 'cookie';

$cfg['Servers'][$i]['user'] = '';

$cfg['Servers'][$i]['password'] = '';

/* End of servers configuration */

$cfg['UploadDir'] = '';

$cfg['SaveDir'] = '';

$cfg['ForceSSL'] = true;

$cfg['blowfish_secret'] = '0(s3$9dea5bfy7ic8wo4l8kn&is3ANFY6NA*CM64';

$cfg['MaxRows'] = 100;

$cfg['DefaultLang'] = 'en-utf-8';

$cfg['ServerDefault'] = 1;

All that being said it looks like PHPMyAdmin does not run on CentOS (It feels crazy just to say that). I know there's something I'm missing. If anyone else has any insight or questions please let me know. I'm happy to experiment.