UPDATE February 2012: The 2012 edition of the Florence Chocolate Festival is being held in Piazza della Repubblica in the city center from February 10-19 2012. For this and other events see the post 2012 events in Florence.
I have just read that the annual Florence Chocolate Festival (Fiera del Cioccolato) will be held in Piazza Santa Croce from Jan 22-25, 2009, from 10am to 10pm. Entry is Free! yay! A report and photos forthcoming after I visit it.
Artisanal chocolate producers (ie, not industrial) from around Italy will have stands with products for sale. There are various planned activities, such as a happy hour with chocolate based drinks, a dinner (by reservation) and a debate about how to make Zuccotto, a ridiculously heavy traditional Florentine dessert. The programme is available in Italian on the official website (unfortunately, the english version of the website is 2 years out of date.)
The location in Piazza Santa Croce is perfect for a combination of food and art. (The fair was in fact previously held at a less central events location, with an entry fee.) So, you can combine a visit to the church of Santa Croce, with its frescoes by Giotto and his followers, and the famous Pazzi Chapel designed by Brunelleschi, with a pick-me up sugar high in the piazza.