Compiz is now available out of the box in OpenSolaris 2008.05 (and also since nevada build 85).

To enable it you just need to start the GNOME appearance preference dialog. It is located on Opensolaris 2008.05 under System->Preferences->Appearance-> Visual effect tab.

In nevada it's located under Start Menu->Preferences->Appearance-> Visual effect tab

If your system is capable of running compiz you'll see the following dialog :

To enable compiz select either the normal or extra settings and that's it :)

If you want to customize your compiz fusion plugins settings you can access the compiz settings manager either via the preferences button or via system->preferences->CompizConfig Settings Manager.

If you are presented with these choices and you know your hardware doesn't support compiz you might be hitting bug 1408.

If you find bugs in either the desktop integration of compiz or compiz itself let me know by logging a bug at defect.opensolaris.org.