When I update the m2eclipse to the 1.0.100.20110804-1717, I can't find the "Show advanced tabs" button in the POM editor.
I just can see the Overview Dependencies Dependency Hierarchy Effective POM pom.xml tabs. And when I go to Preferences->Maven->POM->User Interface, also I can't find the advanced.
The advanced tabs have been removed in m2e 1.0+. Long story short : m2e developers don't think it was worth maintaining them.
This is explained in http://dev.eclipse.org/mhonarc/lists/m2e-users/msg00973.html