1.4 is the current development release.
Migrating to Wicket 1.4
Getting and building it
http://wicket.apache.org/building-from-svn.html describes how to get and build Wicket 1.4.x.
Snapshots can be found in this maven repository and has version 1.4-SNAPSHOT:
http://wicketstuff.org/maven/repository
JDK 1.5
Minimum Java version is now 1.5.
Spring Support
Spring Modules
wicket-spring and wicket-spring-annot modules have been merged. If your project depends on wicket-spring-annot simply replace it with a dependency on wicket-spring.
SpringWebApplication
SpringWebApplication has been deprecated in favor of SpringBean annotation. See SpringWebApplication javadoc for how to setup SpringBean based injection.