ft.com/alphaville

Five Thirty Eight - a site that “combines semi-professional expertise with a dollop of eye-witness reporting,” according to the FT’s John Gapper - has neatly summarised Mark Blumenthal’s comprehensive dissection of the media’s reliance on exit polls as an electoral indicator.
Caveat - Blumenthal’s collection of essays dates back to 2004, but the conclusions remain valid. Some highlights:

Exit polls have a much larger intrinsic margin for error than regular polls

Exit polls have consistently overstated the Democratic share of the vote

Exit polls were particularly bad in this year’s primaries - They overstated Barack Obama’s performance by an average of about 7 points.

Democrats may be more likely to participate in exit polls.

Obama has a strong lead in the (hopelessly preliminary) major polls, and the results of the exit polls in major states will start rolling out within the next hour. Take those results with a liberal sprinkling of salt.

(H/T Dan McCrum)