Why not come up with something yourself? You can start with any one of your previous projects. Think about where you can apply the XML technology in the solution space. More importantly, think about why
you should apply it and how it should be applied.
While you're on this, just remember, in order to get an "all-round" XML exposure, your project must make use of technologies like transformations(XSL, XSLT ), parsing( DOM/SAX ), validating and application components that create XML documents on the fly.
Good luck,
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Ajith Kallambella M. Sun Certified Programmer for the Java�2 Platform.
IBM Certified Developer - XML and Related Technologies, V1.