I think that this way is not really clean, especially if you use maven dependencies.
For each new update of GXT now you have to update the jar file (Automatic and transparent with maven) and the resource folder ( you have to copy manually the resource files this is NOT automatic with maven).
In previous release the resources are well included in jar file so only the update of the jar file was necessary it was very more simple.
The new GWT 1.6 dir structure allows many more features than before. If you want to use them in a good way it is not very good to have images in the classpaths public dir.