GWT 2.4 has been out since September but Vaadin 6.7 is still on GWT 2.3. Vaadin 6.8 is also based on GWT 2.3 and it looks like Vaadin 6.9 will be aswell. Why? Because of the decision the GWT team has made to drop support for Java5.
Vaadin 6 fully supports Java 5 and will also do that in the future – both for the server and the client side. Sadly this means that we cannot move to newer GWT versions as these require Java 6. But there is still hope, Vaadin 6.7 works with GWT 2.4 and we will test all new Vaadin versions also with newer GWT versions. We will stay compatible with the newest GWT version in addition to GWT 2.3 as long as possible.
Vaadin 7 has already moved to use GWT 2.4 and will move to newer versions when available. Compiling the widget set in Vaadin 7 will therefore require Java 6. We still plan to maintain Java 5 compatibility for the server side classes so Vaadin 7 applications will work in a Java 5 deployment environment (Java 6 is a compile time requirement, and only for the widget set). We will explore the possibilities to make widget set compilation easy for those stuck on Java 5, e.g. by making the Eclipse plug-in automatically select a Java 6 version when compiling the widget set even though the project is Java 5 based.
How you can use a newer GWT version with Vaadin 6 depends on your environment. In Maven projects you update the dependency in pom.xml from 2.3.0 to 2.4.0. In Eclipse you need to download the GWT jars and add them the build path (remove the automatically added VAADIN_DOWNLOAD/gwt*).
The decision to still support Java 5 will not affect Java 6 or Java 7 users. Those using a newer Java version can still utilize all the available features of that version.