I believe that a high quality open source Java implementation is important for many reasons, but there is one that overrides all others. I think that having a Java runtime deeply integrated into the Linux application stack both guarantees the future of Java on the desktop and is the only reasonable way for the Linux desktop environment to compete long term with Windows and Mac OS.
The alternative approach (Mono) is a nice technical platform. Politically it just doesn't make sense, though, because it ties Linux's future to that of its biggest competitor. Java, on the other hand is controlled by a company who is a long-term major contributor to open source projects and has committed both programmers to work on them and patent portfolios to help defend them. It is true that Sun could help by open sourcing its class libraries, but not doing that is a commerical mistake Sun has the right to make.
(Note that only neither IBM nor BEA nor any other Java vendor can solve the problem by open sourcing their JVMs, because they all sub-licence the class libaries from Sun and thus do not have the rights to open source them)
Anyway (as though I aren't already overcommitted with "projects on the side"): The Unofficial Apache Harmony Blog is something I plan to keep updated with regular status reports for those of you who don't want to deal with the mailing list traffic.