Security Enhancements and Fixes in PHP 5.2.1 and PHP 4.4.5:

Fixed possible safe_mode & open_basedir bypasses inside the session extension.
Fixed unserialize() abuse on 64 bit systems with certain input strings.
Fixed possible overflows and stack corruptions in the session extension.
Fixed an underflow inside the internal sapi_header_op() function.
Fixed non-validated resource destruction inside the shmop extension.
Fixed a possible overflow in the str_replace() function.
Fixed possible clobbering of super-globals in several code paths.
Fixed a possible information disclosure inside the wddx extension.
Fixed a possible string format vulnerability in *print() functions on 64 bit systems.
Fixed a possible buffer overflow inside ibase_{delete,add,modify}_user() functions.
Fixed a string format vulnerability inside the odbc_result_all() function.
Security Enhancements and Fixes in PHP 5.2.1 only:
Prevent search engines from indexing the phpinfo() page.
Fixed a number of input processing bugs inside the filter extension.
Fixed allocation bugs caused by attempts to allocate negative values in some code paths.
Fixed possible stack/buffer overflows inside zip, imap & sqlite extensions.
Fixed several possible buffer overflows inside the stream filters.
Memory limit is now enabled by default.
Added internal heap protection.
Extended filter extension support for $_SERVER in CGI and apache2 SAPIs.
Security Enhancements and Fixes in PHP 4.4.5 only:
Fixed possible overflows inside zip & imap extensions.
Fixed a possible buffer overflow inside mail() function on Windows.
Unbundled the ovrimos extension.
The majority of the security vulnerabilities discovered and resolved can in most cases be only abused by local users and cannot be triggered remotely. However, some of the above issues can be triggered remotely in certain situations, or exploited by malicious local users on shared hosting setups utilizing PHP as an Apache module. Therefore, we strongly advise all users of PHP, regardless of the version to upgrade to the 5.2.1 or 4.4.5 releases as soon as possible.
For users upgrading to PHP 5.2 from PHP 5.0 and PHP 5.1, an upgrade guide is available here, detailing the changes between those releases and PHP 5.2.1.
Update: Feb 14th; Added release information for PHP 4.4.5.
Update: Feb 12th; The Windows install package had problems with upgrading from previous PHP versions. That has now been fixed and new file posted in the download section.