Another line from the home page, this one above the fold, says "An environment for Ruby". I know it is a language implementation, but to me, that sounds more like an IDE than a language implementation. [Also, the home page doesn't know about 1.1 yet. It has 1.0.1 as the newest version. That can be very confusing.]
Even the 1.1 version of the front page has the "an environment for Ruby" bit high-up, with no reference to it being an implementation until 1500-ish pixels down the page. Admittedly, only people familiar with what Rubinius is are likely to want/need to use Rubinius so far.
This actually puts out a mildly confusing picture. Rubinius uses LLVM to implement the JIT compiler only. LLVM does not provide a performant interpreter, and even if it did, the instructions in LLVM are incredibly lowlevel which would mean even lower performance. For this reason, Rubinius also has a bytecode interpreter, of which the bytecodes are highlevel and associated directly with ruby semantics (send, push_const, etc).
Virtual machine can give the wrong connotation to a Java developer though. It isn't just running compiled classes like in Java. It has to do ongoing runtime compilation.
Sorry, I never said that it couldn't, but I didn't explain what I was trying to say very well either.
Yes, there can be runtime compilation in Java like compilation of JSPs (for example). With Tomcat, Tomcat uses (or used to use- I'm getting old) classes in tools.jar, etc. which uses the JVM to compile and put in the work/... directory which it then loads via classloader, etc. etc. and interprets the bytecode it compiled it into into memory, etc. etc.
I was trying to explain to someone that might want to learn about what is going on with Ruby and Rubinus that unlike the Java JVM whose purpose most of the time at runtime is running precompiled bytecode, Rubinus, MRI, etc. are more often compiling at runtime than Java (at least until Java 7 with JRuby, etc. at which point I'll be completely lost, because I will need to understand how that works).
When I think of a VM in the Java sense from that background, I still think "MOSTLY (and thank you for clarifying) running compiled classes at runtime". I just didn't want the OP to be confused about that, and in process now I probably look like a total dumbass.
I'd heard that JRuby was fastest these days, so it would be great to see some numbers and side-by-side features comparisons.