I have installed the PECL extension igbinary and set the following:

; Use igbinary as session serializer
session.serialize_handler=igbinary

Then, I set the following in Piwik's config.inc.php:

[General]
session_save_handler = dbtable

On the first page load, I would get the login screen:

[[Image(https://dl.dropboxusercontent.com/s/1rk4ka9a84sd075/1%20Login.png)]]

And the session would be in the database:

[[Image(https://dl.dropboxusercontent.com/u/84142/bug-piwik/1%20DB.png)]]

Watch what happens on a simple page refresh, no submitted data (though submitting a login yields the same):

[[Image(https://dl.dropboxusercontent.com/u/84142/bug-piwik/2%20Login.png)]]

The session is stripped from the database:

[[Image(https://dl.dropboxusercontent.com/u/84142/bug-piwik/2%20DB.png)]]

By using the default value for session.serialize_handler (commenting out any changes to it, as I had made), the Piwik DB sessions work as expected. For what it's worth, I CAN use the igbinary session serializer with the default files session handler in Piwik, but not the database.

Nothing was in my PHP error log or I would've included it.

For what it's worth, I'm using PHP 5.3.3 (default shipping with CentOS 6), PHP-FPM from RPM, igbinary installed via pecl install igbinary, using the latest Piwik 1.x.

If I can provide anymore information to help, please let me know!