According to this post, the latest edition of the Zend Core will introduce a bundled free license for DB2 Express.

IBM VP Rod Smith will announce the deal Wednesday at the Zend/PHP Conference in Burlingame, Calif.

The current Zend Core For IBM tied into DB2, as well as Cloudscape, but users had to buy the database to take advantage of it. As of later this year, they will get the Express version of DB2, which runs on computers with one to two processors and up to four gigabytes of RAM, for free, IBM said.

The new Zend/IBM core will support Linux, AIX, and Windows. IBM is billing this as the first major vendor database to be supported by PHP on Windows.

Of course, he mentions, that Oracle might have a difference of opinion there, espcially with the release of their Zend Core for Oracle last week...