Building on the excellent work of Richardo Stuven and Surya Pratap, there are now NuGet packages available for NHibernate.Spatial built against NHibernate 3.3.3:
I expect to maintain these packages in collaboration with Mr. Pratap and keep them up to date alongside NHibernate, including the upcoming 4.x release. Note that the Oracle package is not available at this time. Also note that the MsSql2008 package IS compatible with SQL Server 2012.
The packages are currently marked as pre-release, meaning that to install them you must add the -pre flag in the NuGet package manager console. Here are the commands:
Install-Package NHibernate.Spatial.MsSql2008 -Pre Install-Package NHibernate.Spatial.MySQL -Pre Install-Package NHibernate.Spatial.PostGis -Pre
Contributors to this project are more than welcome. You can fork it here: https://github.com/suryapratap/Nhibernate.Spatial