Sorry it's not possible at this time, but we're considering adding more options to the calculation formulas, so it's good to know that you would find a rounding function useful. It might take a while before we can get to it, but I'll keep you updated.
You can actually use the javascript Math functions in the form's calculations. Try something like this in your formula: (UNITPRICE*QUANTITY).toFixed(2)
Here' s a template form with a couple examples: http://app.formassembly.com/forms/catalog/565