Of course, in particular since in most cases, one must communicate with the GPU using 32 bit floats. (I'm going to guess that this is changing, though.)
Yes, the recent generation of Nvidia GPUs has added hardware support for doubles. This was done to be able to sell hardware for use in financial analysis rather than any use in graphics.
The Panda3D Python library uses double-precision floats, due to it's being a Python library. Currently, these are converted to 32 bit floats which are then sent to the GPU.
