Basically you are just scoping out what the keys are sending and using the arduino to convert keypresses to midi.

Yeah. Which is just what you need if you want to use cheap small usb-midi only keboards (and other simple stuff).

edit: and who knows, maybe this conversion work for more / many / all midi keyboards. Devices like Kenton's midi-usb -> din-midi convertor works with all midi keyboards / other controllers, so the usb-midi protocol should be something all of these have in common.

edit: Looking at the link it does refer to non standard method. Basically you are just scoping out what the keys are sending and using the arduino to convert keypresses to midi. Its not true class compliant midi or anything. You could just as easily do the same with a usb typing keyboard.

yea but that isnt a usb input, is there a sheild with usb host?

Did you look at the socket in ashimoke's pic? Or click the link? "Interfacing MIDI devices with Arduino using USB Host Shield"...

It also depends on how much power that controler needs. For example these new (and pretty cool looking) Korg micro series are said to suck a lot of juice.

edit:
basically something like this:

could work as an arduinoboy with usb input which would allow you to use for example mgb with small usb midi keyboard (such as Akai LPK). It might even be put inside of DMG. The problem here is that these small versions of arduino and the usb host use 3.3 voltage and it'd have to be tested if the controller runs on 3.3. It should work just fine with bigger arduino (such as UNO) on 5V.