I'm going over the Cherry MX spec sheet (http://www.cherrycorp.com/english/switches/key/mx.htm), and I notice one of the mounting options is "metal frame." Are there any keyboards that actually use those? I imagine it would add a ton of weight, and flex less when you're banging your head against it.

It's more commonly referred to as "plate mounting" meaning there is a metal plate surrounding the switches above the printed circuit board. Filco, Leopold, Ducky, Deck and Das keyboards all have plate mounted Cherry MX switches.

In this Geekhack thread there are quite good pictures to illustrate this: http://geekhack.org/showwiki.php?title=Island:16351

Ah, got it. Thanks for listing the vendors that use plate mounting. Keyboard flex is definitely one thing I despise in cheap plastic ones.

There are others, too. I listed those just for example.