Java CSS is subject to GNU General Public License version 2.0, which is problematic when including Java CSS in a commercial application. For insight into this problem, check out Anthony DiSante's blog post "Why the GPL Is Incompatible with Commercial Software."
Before reinventing the wheel by initiating a commercial-friendly alternative to Java CSS, you might want to wait for the JDK 7 release in early 2010. Sun Principal Engineer Mark Reinhold has previously hinted at CSS-based styling (which I believe would be based on Java CSS) being included in JDK 7. Perhaps CSS-based styling will be an official part of this release.