Tom's Hardware Guide has reviewed a couple of cases which can be used as ATX or BTX, depending on the user's choice.
BTX form factor cases are finally available after they were first announced during the 2003 Intel Developer Forum. However, at the same time, vendors haven't yet really committed to this new system format. Instead, they now offer solutions designed for ATX that can also accommodate BTX upgrades. The final choice in form factors is left up to the buyer.
Meanwhile, BTX hardware may be on its way in, but there are also brakes on its development. Only a few motherboard vendors have started production lines for this new form factor. But why wait? These new cases are flexible as far as their form factor is concerned. If they accommodate ATX motherboards now, then they should also work for BTX motherboards later on.