Many VSTi (Virtual Instruments) offer the ability to drag ‘n’ drop MIDI information directly from the VSTi to the project window. Some preferences have changed in Nuendo 4.1 with regards to how Nuendo handles the import of MIDI files so to ensure things “work as expected” go to:
File -> Preferences -> MIDI -> MIDI Import Options and uncheck “Import To Instrument Tracks” and “Auto Dissolve Format 0”.
This will ensure that when you drag MIDI information from the VSTi to the project window it will end up on the track you drag it to without creating a new MIDI track or a new Instrument track in the process.