The topBuffer attribute of panel elements in userInput panels is undocumented. See http://izpack.org/community/berlios-lists-archives/izpack-devel/2007-March/000941.html for a description of the implementation and behavior.
IzPack Maven Plugin
Release 5.0 of IzPack introduces a new implementation of the izpack-maven-plugin currently found at http://izpack.codehaus.org/izpack-maven-plugin/. In contrast to the documentation there, an explicit dependency to the izpack-standalone-compiler is no longer needed.
Please note that also the configuration parameters and their default values have drastically changed, so in case you want to port your current maven build script you should consult the mojo class from the source tree for the current paramaters until a new Maven Plugin site has been published.