I just tried with my D300 and a Transcend 266x (UDMA) card. It's very difficult to tell because when I press the button:

- the camera makes the usual noise
- there is a brief pause while it treats the image
- the card-active green light comes on and goes off meaning the image is saved

The whole cycle takes less than two seconds for a 12Mpix RAW + JPEG.

Note that the camera says that it's internal buffer has enough space to hold 15 images with the current settings so unless I do a very long burst at maximum speed it's unlikely to get stuck writing out the RAW files