Internal keyboard QWERTY; External Keyboard Dvorak
No, only a single keyboard layout can be active at one time and it will apply to all keyboards.  But switching layouts is very simple:  Apple/Command + space.  Or you can no doubt make it even simpler if you want.

Update 2016:  It's possible now to do this with Karabiner, as described at

Different key-maps/bindings for in-built keyboard and Bluetooth keyboard