Pixel based detectors, particularly optical CCDs like what was used in the SDSS camera, are ubiquitous in astronomy. Is there any dead area on the detectors? Not the obvious gaps between each individual sensor, but on the sensor itself. That is, does a typical detector have any gaps between the pixels from microscopic wires laid on the front, alternating doping regions in the silicon, or just areas where a photon can hit and the photo-electron is unlikely to be collected? What fraction of the area is dead?
An astronomical CCD is in principle sensitive across its entire active surface; there are no physical gaps between pixels. The pixels themselves are controlled and maintained by the underlying electronic circuitry, which sets up potential barriers between individual pixels. In practice, there are variations in the actual sensitivity within individual pixels: sensitivity is highest in the center and falls off towards the edges, although it never goes to zero anywhere within the pixel.
This paper (PDF) is a recent analysis of a (frontside-illuminated) CCD; Figure 13 shows the measured pixel response functions (how sensitive an individual pixel is across its surface), and Figures 14 and 15 show the photometric sensitivity map (what the sensitivity is for uniform illumination). In the latter case, you can see that the sensitivity never varies more than about 20% or so, depending on the wavelength of the light. Some of the variation corresponds directly to wiring within the CCD absorbing some of the light, but this has a very minor effect on the sensitivity.
The comparison made in Florin Andrei's answer to a consumer-camera-style CMOS sensor, as shown in the image, is misleading, both because CMOS pixels are physically somewhat more discrete in ways that CCD pixels are not, and because astronomical CCDs do not have per-pixel filters glued on top of them.
(The argument that "there has to be a narrow slice of silicon in between pixels to prevent short-circuits" is not correct for CCDs; the electrical separation between pixels is maintained by the underlying electronics, not by any material barrier. In fact, you have to be able to create temporary "short-circuits" between adjacent pixels in a column in order to transfer the accumulated electrons during readout. That's what the "charger-transfer" in "charger-transfer device" means.)