Nicolas Sarkozy knows that in order to win re-election, he needs to scarf up the "populist" vote that he won in 2007 by poaching on the FN's xenophobic territory and that he risks losing in 2012 because the FN is poaching on his "defend the working man" territory. Since a lot of people are angry at the financial sector, attacking "speculation" seems like a political vein worth exploiting, and Sarkozy is going after it by proposing to establish a "Tobin tax" on financial transactions whether his EU partners like it or not. This was the issue that drove David Cameron out of the euro settlement, and now it risks alienating Monti in Italy and perhaps Merkel as well.
There's a problem, though, in making this a campaign issue: Sarkozy was against the Tobin Tax (especially when implemented by a country in the absence of cooperation from competitors) before he was for it (h/t Mediapart):
Gotchas aside, is the Tobin tax a good idea? I think so, provided that its proceeds are used to finance deposit insurance. And of course it would be a better idea if everyone would adopt it, but that seems impossible while David Cameron remains in office.