Wikipedia says DB2 is no longer Platform-specific:
Historically and unlike other database vendors, IBM produced a platform-specific DB2 product for each of its major operating systems. However, in the 1990s IBM changed track and produced a DB2 "common server" product, designed with a common code base to run on different platforms.
If this is the case, why are there different drivers to connect to DB2 for iSeries and DB2 for Linux, UNIX and Windows? (I'm not sure if DB2 for z/OS has its own driver as well.)