There is a separate roll for hats. That's sure. When you earn a hat the time earned isn't following the 25mins pattern of normal items.
1 roll per 4 hours 17 minutes with 1.19% chance seems to be the formula.
The 0.44% might be close/equivalent to this but it's not correct since it's based on the total numbers of items founds (and hats are independent to items...)
I guess the chance is still the same but you got less chances to find a particular hat, since there are more hats now. It's common logic.
Edit:
Hm... "Valve increased the drop chance to 3.5714% on 9/1/09."
Great
