As OMG is the owner of UML, you should first go to OMG for the official information about UML.
OMG is located at www.omg.org, but you’ll probably want to go directly to their UML home page at www.uml.org. From this site, you can find pointers to the official UML 2 and UML 1.x published specifications and official works-in-progress. For example, the official UML 1.5 specification is found at www.omg.org/technology/documents/formal/uml.htm.
The UML 2 official Request For Proposal (RFP) documents are also there. If you work for one of the over 800+ members of OMG, you’ll also be allowed to look at ongoing works-in-progress.
This site has additional useful background information on UML and some pointers to other informative UML sites.