Exit Poll Failure

Here's what I don't get. With 87% of the precincts reporting, John Kerry is down 100,000 votes in Ohio (2%).

But the exit poll has Kerry leading 51-49 with men, and 53-47 with women. In other words, he should be in the lead. Why are the exit polls, in this state and others, so unreliable? It seems almost all of them overestimated Kerry's share of the vote.

Or, for the tin-foil theorists out there, maybe it's fixed Diebold voting machines. (Note: As far as I know, Ohio banned electronic voting in most of its counties).