It appears to work for me using one of my sample Postgres db's. I am using the latest 1.2.0 build snapshot.
Code:
1a48d10 Created SRC_MAIN_RESOURCES/dbre.xml
11d55ce Updated ROOT/pom.xml
01ee955 Updated SRC_MAIN_RESOURCES/META-INF/persistence.xml
Created SRC_MAIN_JAVA/com/foo/dbretest/domain
f7a6b62 Created SRC_MAIN_JAVA/com/foo/dbretest/domain/T2jason.java
d3401d3 Created SRC_MAIN_JAVA/com/foo/dbretest/domain/T2jason_Roo_Configurable.aj
a6b27df Created SRC_MAIN_JAVA/com/foo/dbretest/domain/T2jason_Roo_Jpa_Entity.aj
5bb0d4c Created SRC_MAIN_JAVA/com/foo/dbretest/domain/T2jason_Roo_DbManaged.aj
cb6b51e Created SRC_MAIN_JAVA/com/foo/dbretest/domain/T2jason_Roo_Entity.aj
ed733e9 Created SRC_MAIN_JAVA/com/foo/dbretest/domain/T2jason_Roo_ToString.aj
com.foo.dbretest roo>
com.foo.dbretest roo> json all --deepSerialize
c3f23e4 Updated SRC_MAIN_JAVA/com/foo/dbretest/domain/T2jason.java
e761e17 Created SRC_MAIN_JAVA/com/foo/dbretest/domain/T2jason_Roo_Json.aj
com.foo.dbretest roo>
com.foo.dbretest roo> web mvc json setup
Created SRC_MAIN_WEBAPP/WEB-INF/spring
829190e Created SRC_MAIN_WEBAPP/WEB-INF/spring/webmvc-config.xml
2451afb Created SRC_MAIN_WEBAPP/WEB-INF/web.xml
ab78e5d Updated SRC_MAIN_WEBAPP/WEB-INF/web.xml
09f58b5 Updated ROOT/pom.xml [added dependencies org.springframework:spring-web:${spring.version}, org.springframework:spring-webmvc:${spring.version}; updated project type to war]
com.foo.dbretest roo>
com.foo.dbretest.web roo> web mvc json all
a58d8fe Created SRC_MAIN_JAVA/com/foo/dbretest/T2jasonController.java
05a141f Created SRC_MAIN_JAVA/com/foo/dbretest/T2jasonController_Roo_Controller_Json.aj
com.foo.dbretest.web roo>
Perhaps you can attach your dbre.xml file so I can test using it
Alan