I have the same problem.
My guarantee for 1 year had not run out and the retailer had a 2 year guarantee on top. I sent the laptop off via the retailer to Sony and was told the battery had gone and it was only guaranteed for 6 months. It only lasted 20 mins on Vaio Optimised and 90 mins on Vista Powersaver when new.
Interestingly on the web, you cannot get the genuine batteries for my FZ38M (BPS8A or BPLA). The US smaller BPSA that fits is available as a generic but even this is not available from many battery websites. This leads me to think that the FZ's batteries are failing much much more than other Sony Laptops.
I haven't treated my battery well as the laptop is always plugged in for the kids to use. That said 10 mins on Vaio Ultimate power (the longest option available - and the screen can be turned down as well) is very poor - and I think I started with a duff battery. As you say.. within the spec.
We should all be told by the shops "Please please run the laptop without the battery in" if this is normal process.
I still love the FZ38M. All the previous screen card problems are resolved, the media buttons are great for the kids on long car journeys, the software bundle is amazing for video and photo editing and the hardware is really good for the price I paid.
I don't know if the cells have a charging counter with a fixed limit. It's probably very unsafe to mess about with that with the provious Sony Battery fire problems on earlier models in 2006.
I agree with the earlier post - make do with being plugged in though I personally will probably pay for a new battery and just use it occasionally.
