The actual arc temperature is usually between 4200 and 5400 degrees Kelvin (7100/9260 f. respectively) from what I've gathered. I haven't found anything about the surface temp of the outer envelope on a single end MH, but from personal experience I can say it's hot enough to singe my skin when servicing parking lot lights! I can't begin to imagine grabbing ahold of a d.e. lamp! On a similar note, has anyone heard of inductive lighting or sulphur lamps? Pretty interesting stuff, can't help but wonder if this is the next wave of lighting...
Well when my hood fell and I stopped it with my hand one of my 175w bulbs did rest up against the back of my hand for a good full second, still have the scar, so it's pretty damn hot.
A little googling though, has the 175w bulb at 475F on the outer surface (SE bulbs), where as a 250w bulb has the bulb temp (ie if it the DE bulb part or the inside of the SE part) at 400C (752F).