I have done some exploratory work with DB2, more out of stubbornness than anything else. There are some minor issues, such as a config modification, but for me the biggest problem was the 8-character table naming restriction. If you can live with that, DB2 is certainly scalable, and the Community version is free. See db2onrails.com for details. In my limited experience, using DB2 means you have to jump through hoops, using MySQL everything just falls into place.
Joe
