DMG uses 5V internally and can run on a 5V power supply without strain, the (normal) 6V from the batteries or AC adapter is converted to  +5V (for most everything inside) and -18/19V (for the LCD's contrast)...

you're good. 

USB provides a cheap regulated 5V, for breadboarding even if you want.  there is a current limitation, but apart from this its your basic regulated 5V power source.  you just ignore the data/clock lines, and use the Vcc and GND.

Ahh, thanks for pointing that out. I'm glad to know I'm not doing any damage now

you're good. 

USB provides a cheap regulated 5V, for breadboarding even if you want.  there is a current limitation, but apart from this its your basic regulated 5V power source.  you just ignore the data/clock lines, and use the Vcc and GND.

How's the noise compared to a normal PSU? Buzzing, humming etc.

It's actually quite silent when plugged into a computer, in regard to humming. Much quieter than battery power imo.

I'm surprised it works as good as it does, though. Last time I checked USB ports supplied 5V of power, whereas the DMG needs 6V.
Although I did notice the slightest of contrast changes between playing and paused states on LSDJ. Playing the track would cause the contrast to jump up a teensy bit - not even noticeable.

I don't know much about backlighting, so whether or not this idea will work with a backlight is unknown to me. My current DMG is totally unmodded and it works fine.
But I did hear about a guy over at 8bc having his backlit DMG totally wrecked. However, it was almost definitely due to the power source he used. Use a standard computer USB port unless you know what you're doing!

I made one of these a while back too. I hacked up a 'car charger' (also not a charger) because the thing barely worked anyway.

http://www.youtube.com/watch?v=294dxP5fWRU

This video is outdated, as I've built a new one with more reliable wiring (and yes, solder!)