Today when i ran nant, i faced the following problem.
[loadtasks] Scanning assembly "NAntCollections" for extensions.
[loadtasks] Scanning assembly "NantTasks" for extensions.
[loadtasks] Scanning assembly "Newtonsoft.Json" for extensions.
Could not include build file 'C:\ServicesTests\tests\env.build'.
Object reference not set to an instance of an object.
The reason is a bug in the older nant.
It has been fixed in 0.90-alpha1 (April 1, 2010)
From the release notes
.NET Framework 3.5
•Modified version (as returned by framework::get-version()) to "3.5".