I use msbuild directly, skipping nAnt. You can call it with a build file as a property and specify the target from the command line. Here is a sample soultion.build file:
Debug
bin$(Configuration)
.\
DALTests\
$(ProjectDir)SSN.sln
$(ProjectTestDir)DALTests.csproj
I call this from a batch file like this:
(in the same dir as soultion.build)
C:\WINDOWS\Microsoft.NET\Framework\v3.5\MSBuild.exe Soultion.Build /Target:Build
You will need the MSBuild Community Tasks dll, just google it.