Dalvik is basically a JVM. A lot of Java bytecode (including non-Java languages) will run on Android.

It does. What would be the rationale for this? Is it just that they didn't want the standard Java ME runtime?

Here is some speculation on why: http://www.betaversion.org/~stefano/linotype/news/110/

It looks like this is good news for anything that targets the JVM though, as it looks like the path to Dalvik bytecode is:

    .java -> [javac] -,-> .class -> [dx] -> .dex
                     /
    ??? ------------'

Has anyone done this, with Clojure, for example?

Very interesting article on a possible explanation. Google is also doing this with GWT.

The most novel thing I've heard about Dalvik is their zygote technique for minimizing app startup time. The zygote is is a Dalvik process that's loaded and ready but for the actual bytecode it will need to run. To start a process it first makes a fork of the zygote, which allows for more shared memory and a more expensive initial setup time for the zygote without any penalty when creating new Dalvik processes.

I dunno if this technique necessitated writing their own VM however.