My name is Emil Ong and I work for Caucho Technology, the maker of Quercus. I thought I'd introduce myself and let the WordPress community know what Caucho is up to with WordPress. Quercus is our cleanroom Java implementation of PHP. It's open source (GPL) and runs PHP code in both interpreted and compiled modes (i.e. the PHP is compiled to Java, then run on the JVM). The interpreted mode version of Quercus is available to run in any Java servlet engine (the Glassfish guys actually have a lot of tutorials up about this), but the compiled mode only runs on Resin, our Java application server, and is only available with a license. We haven't had a chance to benchmark WordPress on Quercus, but we've found up to 4x performance improvement for other PHP apps in the compiled mode over the usual Apache+PHP.
The way that we're approaching development of Quercus is to make sure it's as compatible as possible with PHP, quirks and all ;-). To do this, we've been choosing popular PHP applications and certifying them to run on Quercus. WordPress is one of the top applications that we continually develop for. The next version of Quercus (3.1.5) will be out in a couple of weeks with support for WordPress 2.3.3. Thanks for making a great app! Please let me know if you have any questions about Quercus and/or how it works with WordPress.