Is there a particular reason you want to use DB2? I'm only curious because I've heard good things about it and would like to know if there are features in DB2 that make it particularly interesting.
I wonder if this was ever done or if anyone is using Openacs with DB2. I am especially interested in this as I now work in IBM on a team responsible for enabling Oracle to DB2 migrations. Recent changes in DB2 have made this kind of migration much easier http://www.gartner.com/DisplayDocument?doc_cd=167786 . Arguably recent price hikes from Oracle have made this type of migration more likely!