Is there an official UML certification process?

Depending on what you mean by "official", there are some initiatives towards testing for competency in object-oriented design and analysis, and the UML is part of the package. Sun's Level II Certified Solution Developer test and IBM's Object-Oriented Analysis and Design Test both show knowledge of UML as part of the objectives.

Take a look at these websites:

• Object-Oriented Analysis and Design Test
• Certified Solution Developer
• UML Certification