<p>Is there any GUI-based tools to assist you with writing and maintaining the configuration file? Any code tools to codegen the config file? What are the best ways to make this a little bit easier? Are most people just using Castle ActiveRecord now?</p>

I'm just learning NHibernate myself (from the
excellent Summer of NHibernate
series).  I don't know of any GUI tools, but I thought that I'd point out that if you install the NHibernate schmema files (found in the root folder of the installation) to C:\Program Files\Microsoft Visual Studio 9.0\Xml\Schemas, you then get intellisense help with both the general configuration file and the mapping configuration file.  I found that to be a big help.