4/14/2010 - Updated for .NET 4.0 RTM and feedback from comments.
For anyone who is using NAnt to build their solutions/projects and is also testing out Visual Studio 2010/.NET Framework 4.0 Beta 1, I have included below the .NET 4.0 Framework entry that I added to my NAnt.exe.config file and have been using to successfully compile my projects.
<framework
name="net-4.0"
family="net"
version="4.0"
vendor="MIcrosoft"
description="Microsoft .NET Framework 4.0"
sdkdirectory="${path::combine(sdkInstallRoot, 'bin')}"
frameworkdirectory="${path::combine(installRoot, 'v4.0.30319')}"
frameworkassemblydirectory="${path::combine(installRoot, 'v4.0.30319')}"
clrversion="4.0.30319"
>
<runtime>
<probing-paths>
<directory name="lib/net/2.0" />
<directory name="lib/net/neutral" />
<directory name="lib/common/2.0" />
<directory name="lib/common/neutral" />
</probing-paths>
<modes>
<strict>
<environment>
<variable name="COMPLUS_VERSION" value="v4.0.30319" />
</environment>
</strict>
</modes>
</runtime>
<reference-assemblies basedir="${path::combine(installRoot, 'v4.0.30319')}">
<include name="Accessibility.dll" />
<include name="mscorlib.dll" />
<include name="Microsoft.Build.Engine.dll" />
<include name="Microsoft.Build.Framework.dll" />
<include name="Microsoft.Build.Utilities.dll" />
<include name="Microsoft.Vsa.dll" />
<include name="Microsoft.VisualBasic.dll" />
<include name="Microsoft.VisualBasic.Compatibility.dll" />
<include name="Microsoft.VisualBasic.Compatibility.Data.dll" />
<include name="System.Configuration.dll" />
<include name="System.Configuration.Install.dll" />
<include name="System.Data.dll" />
<include name="System.Data.OracleClient.dll" />
<include name="System.Data.SqlXml.dll" />
<include name="System.Deployment.dll" />
<include name="System.Design.dll" />
<include name="System.DirectoryServices.dll" />
<include name="System.dll" />
<include name="System.Drawing.Design.dll" />
<include name="System.Drawing.dll" />
<include name="System.EnterpriseServices.dll" />
<include name="System.Management.dll" />
<include name="System.Messaging.dll" />
<include name="System.Runtime.Remoting.dll" />
<include name="System.Runtime.Serialization.Formatters.Soap.dll" />
<include name="System.Security.dll" />
<include name="System.ServiceProcess.dll" />
<include name="System.Transactions.dll" />
<include name="System.Web.dll" />
<include name="System.Web.Mobile.dll" />
<include name="System.Web.RegularExpressions.dll" />
<include name="System.Web.Services.dll" />
<include name="System.Windows.Forms.dll" />
<include name="System.Xml.dll" />
</reference-assemblies>
<task-assemblies>
<!-- include MS.NET version-neutral assemblies -->
<include name="extensions/net/neutral/**/*.dll" />
<!-- include MS.NET 2.0 specific assemblies -->
<include name="extensions/net/2.0/**/*.dll" />
<!-- include MS.NET specific task assembly -->
<include name="NAnt.MSNetTasks.dll" />
<!-- include MS.NET specific test assembly -->
<include name="NAnt.MSNet.Tests.dll" />
<!-- include .NET 2.0 specific assemblies -->
<include name="extensions/common/2.0/**/*.dll" />
</task-assemblies>
<tool-paths>
<directory name="${path::combine(sdkInstallRoot, 'bin')}"
if="${property::exists('sdkInstallRoot')}" />
<directory name="${path::combine(installRoot, 'v2.0.50727')}" />
<directory name="${path::combine(installRoot, 'v3.0')}" />
<directory name="${path::combine(installRoot, 'v3.5')}" />
</tool-paths>
<project>
<readregistry
property="installRoot"
key="SOFTWARE\Microsoft\.NETFramework\InstallRoot"
hive="LocalMachine" />
<readregistry
property="sdkInstallRoot"
key="SOFTWARE\Microsoft\.NETFramework\sdkInstallRootv2.0"
hive="LocalMachine"
failonerror="false" />
</project>
<tasks>
<task name="csc">
<attribute name="supportsnowarnlist">true</attribute>
<attribute name="supportswarnaserrorlist">true</attribute>
<attribute name="supportskeycontainer">true</attribute>
<attribute name="supportskeyfile">true</attribute>
<attribute name="supportsdelaysign">true</attribute>
<attribute name="supportsplatform">true</attribute>
<attribute name="supportslangversion">true</attribute>
</task>
<task name="vbc">
<attribute name="supportsdocgeneration">true</attribute>
<attribute name="supportsnostdlib">true</attribute>
<attribute name="supportsnowarnlist">true</attribute>
<attribute name="supportskeycontainer">true</attribute>
<attribute name="supportskeyfile">true</attribute>
<attribute name="supportsdelaysign">true</attribute>
<attribute name="supportsplatform">true</attribute>
<attribute name="supportswarnaserrorlist">true</attribute>
</task>
<task name="jsc">
<attribute name="supportsplatform">true</attribute>
</task>
<task name="vjc">
<attribute name="supportsnowarnlist">true</attribute>
<attribute name="supportskeycontainer">true</attribute>
<attribute name="supportskeyfile">true</attribute>
<attribute name="supportsdelaysign">true</attribute>
</task>
<task name="resgen">
<attribute name="supportsassemblyreferences">true</attribute>
<attribute name="supportsexternalfilereferences">true</attribute>
</task>
<task name="delay-sign">
<attribute name="exename">sn</attribute>
</task>
<task name="license">
<attribute name="exename">lc</attribute>
<attribute name="supportsassemblyreferences">true</attribute>
</task>
</tasks>
</framework>
Again, keep in mind that is what has worked for me. Your mileage may vary. Hope this helps.