Mark Blumenthal urges caution on exit polls. They got it totally wrong in 1992 (see above). The Economist's pre-result primer:

As for performance benchmarks, it may be easiest to think in terms of large round numbers. If Mr Cameron wins at least 300 seats, he will almost certainly be prime minister. If Mr Brown wins more than 200, he will have avoided the kind of rout that could have brought Labour’s long-term viability into question. If Mr Clegg gets his party past the 100 mark, he will have smashed a psychological barrier and earned himself enormous clout in Parliament.