Options for Client Server Communication in Android

What support is there for various middleware technologies in Android?

My personal opinion -- though I do not feel I am alone in thinking this way -- is that only protocols specifically designed to run over the Internet are remotely suitable for use with a mobile client. So, of your list, the only one that I would even entertain would be:

Web services such as XML-RPC, SOAP, REST

Some people have been maintaining an Android port of kSOAP2. However, I get the distinct impression that most Android developers working in this area have tended towards REST and REST-ish protocols. If nothing else, that's what all the fun Web sites and services are using for an API, particularly compared with XML-RPC (old) and SOAP (old and icky).

I have successfully used both the java.net.URLConnection and Apache HTTPClient libraries in Android for communicating with REST-style endpoints -- both directly and through third-party JARs -- with no real Android-specific issues.

How well (or not) do existing Java libraries work when used on the Android platform?

It is difficult to answer that in the abstract. Android implements a substantial subset of JavaSE, but not all of JavaSE, so there's a chance that any given JAR will expect something Android does not offer. Similarly, Android does not use environment variables, command-line switches, or a variety of other things that developers focused on the desktop might have introduced as semi-requirements. So, some things have worked for me with nothing more than a recompile (Beanshell), and some things have worked for me after removing redundant classs (JTwitter), and some things looked like they were going to be ghastly to get working (JavaMail).