Our mileage has been low this summer, too. I usually average of 42-46 mpg, with a peak of 48-51.5 in the spring and fall when the weather is nice. In the summer, mileage often dips to 36-38 mpg, but our Prius is currently getting 32 due to the summer heat.
Per our APS bill (Arizona Public Service, electric utility), this summer has been much hotter than the last couple of years (APS was $425 this month). Not sure about the 2006 Prius, but the air conditioner on our 2002 runs off of the ICE; and the last two months, the ICE runs almost continuously when driving. Thus, the low mileage.
I just replaced the traction battery, and we got our car back today. However, I doubt if the fuel economy will improve with the new battery. I accelerate full throttle off of every traffic light to maximize electric motor usage and reduce gas burned, so I am familiar with the combined electric motor/ICE acceleration characteristics; and there appears to be absolutely no difference in acceleration between the old battery and the new one.
(The failure mode for the old traction battery was "DEAD", i.e., no electric motor assist; and there was no gradual decrease in acceleration prior to the entire hybrid system shutting down.)
You might try using the driving techniques that appear in my previous post, the most important being to pump up the tires. Otherwise, I suspect we'll both just have to wait until October for fuel economy to return to "normal".
By the way, I have purchased 12-volt batteries from the company below on two occasions. The most recent replacement was an Optima Yellow Top, earlier this year. Their prices are very good, and I got one business day delivery in Phoenix, from California. I ordered the battery on Sunday night after midnight, and it was delivered on Tuesday at around 10 AM. Tell Bernie Littman that I referred you.
By the way, I have no affiliation with this company, other than being a satisfied customer (twice):
Note: See instructions on how to test your 12-volt battery near the bottom of this very long Web page.