Actually, they have always been a bit off. Early polls had Dukakis and Reagan in a virtual tie. The "final" exit polling always has to be adjusted to "fit" the actual results, by a few percentage points. It so happens that usually, the election isn't that close so the error goes unnoticed. The last two elections, however, have been reeeal close...
When I say "accurate", I mean that accurate within the unscientific sample suveyed. Exit polls are horribly unscientific-one never hears what the margin of error is the things.
Wow! That's more than a bit off, I'd say. ;-)