I'm working with two VMs, one a web server, one a MySQL database server. In mysql, I added a 'user'@'172.16.1.2' with privileges appropriate for the web site, and that works. The VMs got shipped off to a hosting facility. They got the 172.16.1.X network between the two VMs up, but when they try to "mysql -h 172.16.1.1 -uuser -p" and enter the password, they get an error denying 'user'@'hostname'

Why is MySQL resolving the IPs? I figured it would be quicker to fix that than it would be to look up the specific privileges for that user, add 'user'@'hostname', and then go in and start modifying the config (it's set in settings.php as well as several places in the httpd.conf)

I do have /etc/hosts entries on both VMs resolving both 172.16.1.1 / 2 to known hostnames.

You did specify 'user'@'172.16.1.2' as the enabled user; is the 'client' box that exact IP? ie not just in '172.16.1.X', unless you change it to be a wildcard; see the mysql docs http://dev.mysql.com/doc/refman/5.0/en/