The bottom of my 180G was temepered (found that out after I dropped a router bit on it!). I got that glass replaced already, but prior had ordered a Glass Holes.com overflow kit. So, I went and drilled the back glass anyways and used my Glass Holes kit. I love it... can't hear a thing, completely out of the way, and takes up less room in the tank than an overflow that is drilled on the bottom.

They will send you the kit which has the drill bit included, and they have a video on their site of how to do it. Very easy to do.