In Visual Studio 2008 and earlier releases, one of the key complaints is that the debugger doesn't allow you to debug 64-bit mixed mode (managed and native) applications. In fact, if you try to debug a 64-bit mixed mode application, you will see an error that is similar to the following.
The only workaround is to debug the managed portion of the application using the managed debugger and similarly, debug the native portion using the native debugger.
In Visual Studio 2010, we have removed this restriction such that you can use the Mixed Mode debugger for 64-bit applications. Hence, you will no longer see the above error message.