I have the same problem, and am using the latest versions.
Windows XP
Eclipse: 3.4.1 Build: M20080911-1700
Google Plugin for Eclipse 3.4
Google Web Toolkit SDK 1.6.4
GWT 1.6.4
Hibernate Core 3.3.1
Hibernate Annotations 3.4.0
SLF4J 1.5.6
Any thoughts?