Ipsos MORI poll on expenses
There's an interesting MORI question in Total Politics here: they asked people to imagine that at the next election the candidate of the party they would normally vote for had been caught up in the expenses scandal, would they still vote for the party they wanted to win the election? 53% said yes, 38% said no, they would vote for a different candidate.
That implies a major effect - the majority of MPs would be defeated if 38% of their supporters went elsewhere. I suspect, however, that this vastly over-estimates the effect we'll see in reality. If asked a question about the expenses scandal most respondents are probably imagining the "big ticket" headline scandals, the duck houses, moats and non-existant mortgages. The MPs who had to pay back a couple of hundred quid for accidentally claiming something twice probably don't come into it.
There are 148 MPs seeking re-election who had to repay more than £1000 pounds of misclaimed expenses. Taking the worst cases, there are only 29 seeking re-election who repaid more than £5000. Therefore, in the vast majority of seats, expenses will not be an issue for a single candidate (it may still have coloured peoples attitudes to the government, or the political system as a whole, but there won't be one candidate with a huge expenses issue round their neck). I expect local supporters of the party the MP represents will also be more likely to convince themselves that their MPs was unfairly traduced, or had a good explanation.
There was also a significant party difference - supporters of the Conservatives or Labour party were far less likely to care if the candidate in their own seat was implicated in the expenses scandal (or at least, less likely to vote against them if they are!). Liberal Democrat supporters were far more likely to vote against a Lib Dem MP involved in the scandal, but there are fewer of them anyway (looking at the list of eventual repayments there are no LD incumbenets seeking re-election who had to repay more than £5000, and ten who had to repay between £1000 and £5000).
I would still not expect the expenses scandal to produce more than a couple of particularly unusual constituency results.
(hat tip to Iain Dale, and observe the beautiful example of TFAQ number 4 under the Total politics story)