The tip Qb has posted might be on to something. Normally I would caution against this because li-ion batterys do not get lil happy faces if overcharged. I would think htc would have built in circuitry so this doesn't happen but who knows.
Yesterday I had a lite to moderate day of use with my phone. My battery life was suffering after working great the day before, and I even had wifi and bt turned off. I powered my phone off, and let it charge from the power port of the usb on my laptop (it supposed to be better for charging...different than other usb ports) for about an hour or so. No green led or anything. A coworker came by, was interested in the phone so I took it off charge, powered it up, turned on all the bells and whistles, BT - Wifi - data always on - etc and we played with it for a bit. Put it back on the laptop cable while on for the next hour or so, didn't seem to get past 70% charging.
Went home, put it on the wall charger overnight, left all the bells and whistles on, and I've messed with the phone a little today. Installed two updates to apps, viewed a couple emails, sent a couple texts, made a phone call. Less use than yesterday but I haven't shut off any bells or whistles yet. I even connected to the local wendys free wifi spot at lunch lol.
From the best of my knowledge using the stock battery indicator, the phone is sitting at 80%. Up time is 23:31 and awake time is 3:13. I would say it seems to be performing better than it has on any other day I've used it.
I wonder if there is something in the charging circuitry that, like has been suggested, is not charging the battery all the way? I wonder if thats why they say to charge the phone all the way before even turning it on?
I only charged mine while off for about an hour though. Still I am intrigued. I use li-ion batterys for other things, and I know they seem a little flat and a little faster to go dead if on less than a full charge, where a full charge gives what I expect. Maybe this is a similar case here.
Oh, I haven't stopped the calender service or anything since the full power up yesterday, I did before I powered it off, but when I turned it back on it still was running same as before so I didn't mess with it again. The only other appreciable change was that I choose the 12am to 6 am time or whatever it is for backup assistant to do its thing.