What is a backlit CCD sensor, and why might I prefer it to other kinds? I've heard Astronomers talking about backlit CCD sensors, and talking about how much better they are than other types. What are they, why might I want to get one, and what are the pros/cons of this compared to related technologies?
 A: A backlit sensor isn't really a new kind of sensor, it's just a different arrangement of the imaging elements to allow more light into the sensor. From Wikipedia:

A traditional digital camera sensor
  consists of a matrix of individual
  picture elements. Each element is
  constructed in a fashion similar to
  the human eye, with a lens at the
  front, sensors at the back, and wiring
  in between. The front of the detectors
  require an active matrix to be placed
  on their front surface. The matrix and
  its wiring reflects or absorbs some of
  the incoming light, thereby reducing
  the signal that is available to be
  captured.
A back-illuminated sensor moves this
  wiring behind the sensors, similar to
  a Cephalopod eye.

They're not without issues though:

Moving the active matrix transistors
  to the back of the photosensitive
  layer normally leads to a host of
  problems, such as cross-talk, which
  causes noise, dark current, and color
  mixing between adjacent pixels.

Backlit sensors improve the low-light performance of a CCD chip, reducing the ISO needed for low-light conditions (and thus reducing noise). This is a clear benefit to astrophotographers, allowing them to use shorter exposures with less noise to get equivalent shots.
