Committed to a feature branch:
http://github.com/beberlei/doctrine2/tree/ibmdb2
The DBAL tests work "ok" for this, however it segfaults on allmost all ORM tests. Its getting to annoying to work with this, does anyone want to pick up from there?
Linked maybe, it seems IBM DB2 either does not allow the DEFAULT key for columns, or you have to specify a valid default value. I have to investigate this problem more...
Got PDO_IBM working, just one little problem left to tackle 
https://www.ibm.com/developerworks/forums/thread.jspa?threadID=325698
PDO_IBM is completed, there is only one bug in PDO_IBM that needs to be resolved for the full support (http://pecl.php.net/bugs/bug.php?id=17199), i.e. CLOB/BLOB values don't work correctly.
You can use string types instead, which allow up to 32000 chars.
Still missing is the support for IBM_DB2 Pecl extension.
Additionally both drivers allow to access Informix databases also, which is not supported yet at all.
There is now support for the IBM_PDO and the IBM_DB2 extensions in trunk of Doctrine 2