Anecdotal, but...
I know a number of people who WANTED to vote Lib Dem but, being in a seat where the choice was between Labour or Tory, decided to vote Labour as the lesser of two evils.
Leaving aside for the time being the fascinating question of which of those two evils really is the lesser, proportional representation would have made the vote they wanted to cast count more, so they'd have been more likely to vote with their conscience instead of having to adopt a more tactical approach.
Or, to put it another way, the percentage of the Lib Dem vote was artificially low (by a minimum of two votes) because of the inherent unfairness of our FPtP system. It's a disgrace, but while the power keeps being passed between the two sides that the system is biased towards, it's unrealistic to hope for any change.
Perhaps now, with the power in the balance and Clegg's thumb on the scales, we may see some movement... *dreams...*
(I'm not scared of a coalition government. Look what a strong majority brought for the last few cycles...)