Can I use the RMI registry for the commercial purpose without having to pay Sun royalties or licensing fees for the same? My guess is I don't since it is a part of the JRE, but what is the definitive answer on this?
When you download the JRE or JSDK you are presented a license that is applicable to all the downloaded software, including RMI since it is a part of the JSDK/JRE archives.
All the core APIs and standard extensions (javax.* packages) are free for use within your products.