Note a few things on the data sheet:
First, the package has an "EPAD" on underside for cooling purposes. The cooling is through the circuit board. That's how you can dissipate significant power in such small and "flimsy looking" package.
Secondly, the chip has also thermal protection and other protections.
I have not personally used this chip, but I wouldn't be too worried: I have used an power op-amp in the same type of package, and even placed a cooling fin on the opposite side of the circuit board. No trouble at all, actually I over-designed it, and the chip was cool as a cucumber. Could have skipped the cooling fin and thermally conductive epoxy glue....
Often, if not loading too heavily the printed circuit board may be good enough heat sink alone for these packages. You should calculate the expected max. dissipation and then calculate the temperature rise for it - with circuit board copper cooling it, or with an extra cooling, if absolutely required.
Look also at ST's web site for their evaluation board. There you can see the cooling "contact" area as well, using the circuit board only:http://www.st.com/internet/analog/product/248592.jsp