It seems that exception javax.faces.validator.ValidatorException thrown by org/jboss/seam/ui/validator/EqualityValidator.java (backend of s:validateEquality tag) during comparing values is not caught by JSF and the error message is not displayed.
The same example works fine with JBossAS 5.1.0.GA.
Steps to reproduce
1) deploy "ui" example from Seam distribution to JBossAS 6.0.0.M1
2) go to s:validateEquality or s:validateEquality(2) link on main page
3) enter different values
4) after submitting you can see that "OK" is displayed which is wrong