I have 5 batteries, X1 TB47 and X4 T48's. I bought the X4 TB48's at the same time in April, and used them all twice. When I come to charge them for the 3rd time I noticed one of them (my No2 battery) seemed to be dead as a door nail. Mine also won't light up when I press the power button, nor will it charge. I fly with my critical battery warning set to 30%, and land typically with 25%-30% left (by the time I've landed) so I know there's a safe amount of power in it (like the others). Apparently its in hibernation mode, and not as dead as it appears to be.
Good News! The latest firware update posted 26/5/15 corrects this from happening. What from happening? According the release note, its main update it to prevent a limited numbers of batteries from "not coming out of hibernation mode when stored for prolonged periods of time when storing less than 40% power". So, our batteries probably aren't dead, they just need the latest firmware released 26/5/15 to stop it happening again.
Bad News, However! Since the battery won't power up in the first place, not even in the Inspire, nor will it charge, theres absolutely no way of installing the firmware update on it to cure it or stop it happening again. So the firmware is just prevention, not a cure.
Result: Since there seems to be an identifiable problem, and a valid warranty, I will still be sending mine off to be replaced. When I hopefully get a new one I will update the firmware immediately (to prevent the new battery suffering the same fate). I'm not so sure about taking it apart myself and messing under the white cover for fear of damaging it permanently, however, if I didn't have plenty of spares ones (or a valid warranty) to keep me going I may have been tempted to give it a go.
Final thought: I just hope its a fairly quick process to getting a replacement.