I ported Phorum Bulletin board from MySQL to IBM DB2 9.5.2 or higher. Now you can run PHP based bulletin board solution on your server backed by IBM DB2. It is fully working with exception that maximum message size is 32KB, which is more than enough for practical purposes because you can attach large files to message.
Porting was quite easy, exception was replacing MySQL fulltext search engine with db2 text search extender (now included in every db2 edition) and porting LIMIT, OFFSET MySQL feature. I used Oracle compatible ROWNUM. Rest was mere removing mysql specific sql such as query hints, USING clause in JOIN and remove of number quoting like '22'. DB2 PHP driver is quite similar to MySQL, so on PHP side there were no unexpected troubles.
Phorum as typical PHP application is not using transactions at all, so database writes are kinda slow in DB2 and you might run into data inconsistency (needs to be fixed from time to time via administrative interface). Explaining benefits of database transactions to average PHP coder is simply waste of time because these people care only about 1 thing: speed and as we know transactions often introduce locking which lowers database throughput.
DB2 phorum is downloadable here, feel free to report any remaining bugs.
I have idea for port to DB2 program. IBM should sponsor people porting opensource software to run on DB2. It should start like bounty hunting: Port to DB2 software like Drupal, Wordpress, MediaWiki and get $500 reward. It will have great impact because people can try DB2 Express on application known to them. There is no need to port lot of applications, about 5-10 will be enough.
In my spare time, i will target Wordpress next.