By default, Hudson plugins can also run under Java 5, but only if the plugin author is careful not to call any of the new Java 6 APIs.
Unless a plugin requires the new Java 6 features, the animal sniffer plugin should be added to the plugin's POM to enforce Java 5 compatibility. See the blog entry compiling-jdk6-and-running-jdk5 for instructions.
It may also be necessary to increase the memory available to Maven. For example, set the MAVEN_OPTS environment variable to Xmx1024m.