I have upgraded to 7.5.9 and I still get the same behavior when attempting to point at the new DB server. One minor difference though... the mysql.inc.php line in the error returned has changed to 61, but that just seems to be an error reporting call not the root error and is probably not relevant to the issue (though it would be nice to see farther up the stack at what actually puked.) Same MySQL error in the log though.
The darnedest thing though as I've never had any issue changing DB server before and every other MySQL dependent app was a seamless switch. This is a production server so I can only take quick jabs at it. Perhaps I'll try it again this weekend and fully reboot the web-server or something after making the switch.
It's almost like it can't talk to the database, but as mentioned, I've pored over all that before, verifying permissions and can even access it using our one-off forums reporting tool (we use forums for support and I developed a read-only tracking tool that watches for unanswered threads.)