[jeremyshaw]

general rule of thumb, if it is cold to the touch then it is metal (aluminum for phones)

Assuming it hasn't been conducting heat immediately prior (leg, sunlight, desklamp) :P
But yeah, my Titan's (back) top ~60% is cold in the morning (metal), while the bottom part with the "Windows Phone" logo is warmer, and is immediately noticeable as plastic.

[pecuevasb]

That back of the HTC TITAN, both the international and AT&T version, is one piece of aluminum. It isn't plastic.