4500 miles, 3 1/2 months. My mileage is higher than it would've been otherwise because of a family wedding out of town, helping my mother move (again, out of town), and one hurricane evacuation. No, I'm not driving more. My old vehicle was 120K+ miles and the only thing that kept me from driving it more was worrying about mechanical failure... aaaand the 13MPG it was getting lately on my 5 mile commute. The 45+ MPG I'm getting now is definitely a bonus, and I'm trying to keep my miles down for warranty reasons. So, definitely saving gas/air vs. my old car.
Yes I'm driving more because I have the Prius. Here's why. Before I had the Prius (1990 Ford Probe), we took my wife's Windstar when we were on trips together because it has airbags and a working A/C. Her mileage is also less than half of the Prius'. By using the Prius when we are together, there is less money spent on gas and less fossil fuel effect, than if I used the Prius strictly for commuting and we went on trips in her Windstar (which she also uses for commuting).
I wouldn't really count that as driving more. You are just changing which vehicle you use. As a household, you're really driving the same distance.
Many of my trips in the Prius are short (5-10 minutes) which yield disappointing mileage. When I get to make a longer drive I get so excited when the MPG averages zoom way up, I can hardly stand to STOP driving. Yes, I sometimes go around the block a couple of times to get a couple of extra tenths....