Somehow I doubt that we'll decide to drop support for 65.5% of our users (roughly 26 million websites).

http://wordpress.org/about/stats/

It's funny how many people say WP should drop 5.2 compatibility. It's certainly possible to push the hosts towards using 5.3, but the fact is that the user base simply isn't there yet.

Wordpress has adopted the PDO standard. It will be in WP 3.6. http://core.trac.wordpress.org/ticket/21663

That's the aim. Whether we actually get land it in 3.6 or not is another issue, since 5.5 isn't stable yet, so it's not really an issue just yet.

(Personally, I want it, but I can see there being friction, given the possibility of breakage.)

Do you have a source for PHP 5.5 dropping straight MySQL support? I'd like to read more.

That doesn't support the claim of no support for straight mysql connection in 5.5. It says they are going to focus on documenting how to use mysql with the newer modules instead of the old one, and that they will look into possibly adding deprecation notices for the old module in 5.5. Which means it will obviously still be there, and still work.

mysql_* functions will still work, they will simply be marked as deprecated from 5.5 forward:

https://wiki.php.net/rfc/mysql_deprecation

As of 5.4, mysql_* were officially discouraged via the docs.

Maybe in > 5.5.* mysql_* functions will be officially removed from core?