Lets say you have a file (image.png) and you want to convert it to a BMP for whatever reason. It seems in OS X 10.8 you can still do this, but the options are a bit hidden.
This link gives the hint to do this. Within preview you need to hold the “option” key first so you can choose “save as”. After that you still need to hold the option key when you use the ‘format’ dropdown so you can select the BMP format. If you don’t do this, you will not see the BMP format.