I can back my database up via Infopop Threads and via a command line mysqldump.
Problem is when I try to use PhpMyAdmin, it hangs if I include the messages table in the backup. Otherwise, it works fine. Any suggestions?

When you say hangs? Do you get errors - or does the browser just freeze up?

Have you tried optimizing or repairing that table to see if that fixes the problem.

How large is your posttable? Perhaps it is just taking awhile to backup. I would definately try optimizing and/or repairing the table as Josh suggested.

It times out and I get a page not found error.

A few thousand posts. I optimized and repaired. I'll keep trying.

This won't work if you sserver is in safe mode. But in the config file for phpMyadmin there is a max execuation time. Usually this is at 300 by default. Set this to 0 to turn the limit off. However if your server is in safe mode - this won't have any effect. But it's worth a try.

2.52 is supposed to do a better job of exporting large tables, let's cross our fingers as I've had the same problem

I recently upgraded from 2.5.0 to 2.5.1, and it looks like the capability to export to a local file was added, by specifying a value for $cfg['SaveDir'] in config.inc.php.