While starting I get several log messages saying “The message source is unable to find message code…” and the client won't start up.
This is an Eclipse Issue with the runtime configuration. See the menu entry Project → Properties → Java Build Path, on the tab Projects remove the project “spring-richclient-support” and add it again. After a rebuild everything should work flawlessly.

How can I use Java 1.5 features in a Spring-RCP project?
The use of Java 1.5 can be configured in the pom.xml, just add the following to the plugins-section:

<plugin>
	<artifactId>maven-compiler-plugin</artifactId>
	<configuration>
		<source>1.5</source>
		<target>1.5</target>
	</configuration>
</plugin>