Transparent Gateways (formerly Heterogeneous Services -- HS) lets you interact with non-Oracle databases as through a database link. Oracle does not even know that the remote DB is not Oracle.
Transparent Gateways do require a special license. You will have to work with your Oracle salesman to obtain pricing.
For more information on Transparent Gateways, please read the Oracle docs for DB2.
This was first published in August 2007