symfony 1.4.17 has just been released.
Here are the changes from the CHANGELOG:
-  added some tests (closes #3237, patch from Stephen.Ostrow)
-  fixed a notice on PHP 5.4 (closes #9985, patch from bshaffer)
-  fixed Notice with PHP 5.4 (closes #10003)
-  fixed error in Debug mode from sfDebug.class.php when myUser implements sfSecurityUser (closes #9996)
-  reverted  because of side effects (refs #8348)
-  fixed test for PHP 5.3 (patch from pylebecq)
If you've checked out a copy of the tag from Subversion you can switch to the latest version:
$ svn switch http://svn.symfony-project.com/tags/RELEASE_1_4_17
If you are using the PEAR package you can update using the pear command:
$ pear upgrade symfony/symfony-1.4.17
And as always, don't forget to clear your cache after upgrading.