I know what you mean - most glassy effects seem to concentrate on fancy curves, and not the picture-frame flat glass that you want. That said, the following tutorials, slightly adapted, might give you what you need:
http://www.rw-designz.com/tuts/ps/block-1.php
http://www.solinari.net/view/tutorials/glassbox/
http://www.heathrowe.com/tuts/glasseffect.asp (textured glass)