David, I've heard/read that the main battery packs in the hybrid cars (mostly Nickel-metal Hydride) have had super reliability - some are 10 years old and still doing fine. I think the hybrid manufacturers have done a great job in monitoring the battery pack - mainly insuring it doesn't get too discharged or overcharged and also battey temp, etc. So, I wonder if your pack is really going bad.

I've heard that getting a used pack from a salvage yard usually is okay but it really helps if they can tell you how old the car was and how many miles it had when it got wrecked. Anyway, I think I'd watch things and if your MPG hasn't dropped, I'd wait.

My good buddy and his wife have a Civic hybrid (maybe an '06) and his wife gets over 50 mpg in city driving (sometimes 55 mpg!). But she has learned the hypermiler techniques and is manic about them. She chews out my buddy if he drives the car and drops the ave mpg.