If you are using SmartGWT, it includes a RestDataSource which provides a pre-built JSON or XML protocol that you can implement in .NET. This series of articles from our public wiki shows how to build an ASP.NET MVC backend for RestDataSource that implements all 4 CRUD operations, database transactions and support for SmartGWT AdvancedCriteria (using NHibernate):
http://wiki.smartclient.com/display/Main/Integrating+with+ASP.Net+MVC
In this particular case, SmartClient was used rather than SmartGWT, but all the backend code is identical (SmartGWT just provides GWT support for SmartClient; it's the same code under the hood). The front-end code is trivial and easily translated to SmartGWT.
This code on the wiki is all free, and so is the editing of SmartGWT needed to build the front-end (it's open source under the LGPL).
