Has anyone had to do this yet? At what mileage and age did it require replacement. Was it under warranty or out of pocket. I'm just curious how long these last. By my understanding, the technology is similar to a cell phone battery, which for me, only last a few years.

Don't panic, the HCH is designed to maximize battery life, unlike a cell phone.

I hear reports of about 7 years predicted from battery research articles.
I think Honda warrants them for 10/100k in CA so we can hope for better.

HCH's aren't that old yet, but Insights are pretty close I think.

Yeah, but remember that it isn't a FULL 10 year/100K warranty. After a certain amount of years (I forget how many), it becomes a pro-rated charge.

I called my Honda and they said that they have only had to do it one time and the cost was $2,400 and that was it was very recently.

From Honda.com

Q: How long does the battery pack last?
A: The battery pack is designed to last 10 years under normal driving conditions. Its battery pack comes with an 8-year/80,000-mile warranty and all emissions-related equipment comes with a more extensive warranty. See your dealer for details.

I do pretty well in my hch without using the assist very much because it only comes on when I floor it or a steep hill. So... what if, when the batteries wear out, I just don't replace them. Still taking advantage of the small engine, the cylineer shutdown and such. It might still be a great FE car with bad batteries. ???

Probably, but without the IMA accelleration would almost be dangerously slow in some situations. I think it would get around though.

When I bought my 03, the story from the salesman was that while the packs were currently about $3000 (in 03) to replace, Honda projected that by the time the pack needed replacement (2013) the cost should be around $500-600. Seemed reasonable enough to me even if he was only half right. A $1000 repair on a 10-year old car is not totally out of the ordinary.

*** Retired after 65,000 outstanding miles ***

The number 1 concern from people who are looking to buy a new car and considering a hybrid seems to be, "How much will it cost to replace the battery?" With people like Rush Limbaugh telling his followers that it will cost $8,000 to replace the battery pack, public misperception will always be there. His followers will continue buying a new Pontiac or a Monte Carlo and end up paying more for repairs than the HCH owner will pay.
I guess it boils down to trust. I trust Honda and Toyota know what they are doing because of their track record for quality, regardless of what makes the car go forward.
The hybrid dissenters are usually the Buy American crowd that can't admit that Honda/Toyota/Nissan are kicking their butts (e.g. Rush Limbaugh & followers).

In my warranty book for the 2006 HCH it says;
Integrated Motor Assist System (IMA)
Battery Assembly (included all internal components) 10/150

it also stipulates Federal & PZEV, also states, Ca. Ct, Ma, ME, Ny, Vt.

Why buy extended warranty?
To cover the Trans, and ac components I guess.
I mean 10 years? 150,000 miles is a long time folks.
Are you going to own that car that long?

I firmly believe the battery and components will last a very long time.
I am planning to keep my car 4 to 5 years.

This is a common misunderstanding when reading the standard warranty book. And Honda is really to blame for not making it clearer. In the book under the heading "Battery", they are talking about the 12V battery under the hood, not the IMA battery. But most people when they read this section think it's about the IMA battery. Even dealers believe and tell people the IMA battery is pro-rated. No, it's the 12V battery that's pro-rated after 3 years, just like most other car batteries.

Just keep reading till you get to the VERY last page of the warranty book where it lists the specific hybrid-IMA warranties. It lists the states (including California) where it has a full 10yr/150K warranty.
In the rest of the states it is a full 8 yr/80k warranty.
No pro-rating in either case.