First off - great job! I love the integrated unit testing. I was using TestDriven.net before, but I always missed the colors of NUnit and now I have them again.
This is just a minor comment but it relates to all of the dockable windows like Find Results, Unit Test Runner, and File Structure. These windows really need icons to help distinguish them when they are hidden by other tabs. See attached screenshot.
I may have found a bug in the Unit Test Runner area. When I try to run a NUnit test from the IDE, I get a system exception that it cannot find an xml file, MyXmlConfigFile.xml (names have been changed to protect the innocent...). This file is required by the test and is copied to the build folder as a custom build event. The file is definitely there. I can run the same test with NUnit 2.2 with no errors. Based on the exception text, you're launching the Unit Test Runner from the IDE application path and not the project output path (debug, release, etc.):
Read MyTestFailed: MyXmlConfigFile.xml System.IO.FileNotFoundException: Could not find file "C:\Program Files\Microsoft Visual Studio .NET 2003\Common7\IDE\MyXmlConfigFile.xml".
File name: "C:\Program Files\Microsoft Visual Studio .NET 2003\Common7\IDE\MyXmlConfigFile.xml"
at System.IO.__Error.WinIOError(Int32 errorCode, String str)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean useAsync, String msgPath, Boolean bFromProxy)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share)
at System.Xml.XmlDownloadManager.GetStream(Uri uri, ICredentials credentials)
at System.Xml.XmlUrlResolver.GetEntity(Uri absoluteUri, String role, Type ofObjectToReturn)
at System.Data.DataSet.ReadXml(XmlReader reader, Boolean denyResolving)
at System.Data.DataSet.ReadXml(String fileName)
at MyCompany.Data.MyTestData.MyXmlConfigFile..ctor() in C:\MyApplications.root\MyApplications\DataObjects\MyTestData\Resources\MyXmlConfigFile.cs:line 43
Keep up the great work!