The DOE ran the Gen 1 Prius out to 160,000 miles, and while they found the battery capacity degraded by ~50% there was no significant drop in mileage. The batteries in the Gen 2 & 3 are improved over the Gen 1, and should be even better. Under most normal circumstance, mileage is not a major function of battery capacity. http://avt.inel.gov/pdf/hev/end_of_life_test_1.pdf
Be aware that the 12v battery (like any car) has a life of about 4 years (like any car) and that some of it's failure modes can effect mileage, as the car is trying to recharge it constantly. So if you 12v battery is over 4 years old, have it checked out. (unlike any car, the prius does not start slower as the battery ages, as the 'starter' runs off the 201 v battery)
