I think it'd be a bad idea, burning metal is one thing, that's supposed to work, but plastic is another. Burning plastic can give off noxious chemicals which while they may not be damaging to you in small quantities, they can be to your phone. You also weaken the back of the phone, and the engraving would not be like metal, so you could easily get more scratches and CRACKS, in their. Bad idea if you ask me.

check out http://www.adafruit.com/laser/

they also have forums.

the basic gist is that laser etching plastic is ok, be wary of chemicals and don't etch anything white as you tend to get an ugly burned brown color as opposed to a clean line.

a black iphone should be okay though.

hope this helps

I would say start with the lowest intensity setting then keep going over it if it's not enough. Of course, that would mean you'd have to leave it in place so not to adjust the alignment in the engraving devise. Good luck.

I have taken Electronics as a subject for 4 years and I am currently taking it for GCSE. We always house circuits in plastic cases and these are either vacuum moulded or laser cut. The laser cutter is perfectly safe for cutting plastic provided it has a ventilation device, which is really should. In the program we use, Black lines engrave on the surface of the plastic, red lines cut through and blue lines rasta. I have laser cut on Acrylic and Formex.

Also, Apple provide a laser engraving service for iPod or iPhone on purchase. You only get a few lines of text but it shows it can be done.