<p>I'm trying to get the <strong>GWTShell</strong> mode to load my <em>context.xml</em> file in which my database is described.</p>

<p>The only usable info can be found <a href="http://gwt-maven.googlecode.com/svn/docs/maven-googlewebtoolkit-plugin/configure_tomcatlite.html" rel="nofollow noreferrer">here</a>, but this doesn't seem to work for the <em>context.xml</em> part.</p>

As of version 1.4, I have been running all my server side code, in my container of choice (Glassfish) and hooking up the GWTShell to that. Are you using Netbeans, Eclipse or something else? The Netbeans plugin gwt4nb does this for you out of the box, you just have to start your web project in debug mode. I'm sure the GWT plugin for Eclipse does the same thing.
I realise this doesn't directly answer your question -> but my question is, is there a reason you're trying to get GWT to pick up your database settings and not just running your project as normal instead. I find this much better and robust way of running the GWTShell.
Edit: Sorry I don't really use Eclipse, so I can't help you with plugins for it. I find Netbeans far superior for J2EE/web type projects. It's a bit slower, but far more functional. The plugin for that is called 'GWT4NB', it's free and it will set up your ant script in such a way that you just have to right-click on your web project and choose debug. I can understand if you don't want to switch IDEs though.