Happen to work in retail where I was given the display DSi.. it will not power off for anything. I know it is not something broken on it. The rep told me they are all that way. Maybe it is something physically built into the display units where you can't power it off, but I can't find any evidence on the net of this. Find it hard to believe Nintendo would manufacture different models so the power could not be turned off. Maybe it's something in the firmware? I updated firmware to 1.4.3U. Still won't power off. Does anyone happen to know?

 

A friend of mine had a problem with powering off on his dsi too. Apparently when he lent his dsi to his lil cousin, he kept the dsi on for almost a week(charge when battery was low). We fixed it by letting it run until battery dies.charge it again, repeated that a few times until its fixed. Maybe try that?

 

What does the dsi do if you remove the battery and reinsert it?
Does it power on automatically?

 

Try holding the power button for a few secs to force it off. When my dsi froze after hitting the floor I did this and it worked.