Another update:  I switched to a non-backlit gameboy and usb power.  No heating up and cart behavior is back to normal.  So, moral of the story is careful how you combine your mods I guess. 

Probably but not necessarily related: The cart I've been using with this arduinoboy has become very unstable.  It rarely boots to either page (64M cart) even after multiple formats and writing different roms.  I've written multiple copies of mGB to each page which makes the boot menu come up and that seems to be working ok.  Don't have any confidence in it tho. 

I had backlights turn on when 2 gameboys are connected with a link cable, and one was powered... I don't think it could hurt that much.

There is a diode at the power pin of the dmg's link port pointing out to block positive voltage entering the gameboy.

or mod the dmg to be power BY the arduinoboy

Makes sense.  Since I know current is moving between the arduinoboy and the dmg maybe I could mod it to be powered from the link port (it's a xiwi model, i'll get his take on it too).

sounds like its pushing 5v over the gamelink pin. If the aboy is self powered you can probably just cut this pin out if you are worried about it. I would think it would only be a problem in a surge situation.

I have a backlit dmg that I'm using with an arduinoboy.  The arduinoboy is powered by an ac adapter.  If the arduinoboy is plugged into the dmg (and the power adapter), the backlight stays on (dimly) even when the dmg is turned off.  Should I worry about it or no biggie?