Oh my goodness, why you want to draw on it is beyond me, but I guess so. Maybe try a test spot and make sure it won't bleed. Make me feel better and tell me your last name is Picasso.
if you really wanna draw on it.. get yourself a full body skin and draw on that skin. sharpies have the ability to smudge and look like total **** when you get sweaty hands on it.
and trust me.. you will get sweaty hands when you use that phone alot (or any phone for that matter)
If your phone becomes too hot (say on a summer day in your pants pocket), it is possible that the ink will no longer be "permanent" and rub off onto your pants.
Also, any exposure to organic solvents will allow the Sharpie to bleed. If you do not understand what I mean by organic solvents, read this article.
Should be fine I got my battery cover autographed and it eventually just rubbed off anyway so no there shouldn't be a problem and besides permanent markers aren't really permanent anyway