Hi Radik,
As I read documentation and tested some verions Moodle (including the newest version Moodle 1.5 alpha), Moodle hasn't supported IBM DB 2 yet.
MySQL and PostgreSQL are fully supported. Some works on Oracle and MS SQL have started. Documents for developers wrote "Moodle 1.2 will use a new method of database-independent XML schemas" (change 1.2 to 1.x or 2.x 
) and will work for all kinds of databases.