Lithium-Ion batteries have a "break-in" period. It takes *about* twelve full charge cycles for them to reach full storage potential.
As for task killers, you do *NOT* need anything to kill tasks on a schedule. I know everybody freaks out when they see the list of "loaded" applications, but you have to understand the difference between something that "loaded" into memory and something that's actually "running".
Android leaves applications you use more frequently loaded into memory so that it doesn't have to reload them all over again every time you tap the icon. "Cold launching" an application for the first time requires loading all the graphics and everything else from the storage medium and requires *significantly* more battery power than is required to merely "hold" it in memory. When Android detects that it needs more memory than is available, it will automatically shut down app(s) that haven't been used in a while.
That said, I do still have a task killer installed on my phone, but I use it on a manual basis *only*. Personally, I recommend "System Panel", since it has a readout very similar to Windows Task Manager and will show you the CPU usage of each app as a small, gray bar to the left of the application listing. If the phone is acting sluggish, I go looking for apps that are using CPU and shouldn't be and I can manually force them to shutdown. If you pay the $3 for the pro version, it will also allow you to record historical data that can be useful for tracking down applications which are constantly popping up and draining the battery.
And everyone's correct at the widgets affecting battery life. It's the best/worst thing about Android. These phones will sync so much different kinds of data, but usually the default settings are bit frequent. Another big problem I see all the time is people leaving WiFi and Bluetooth on all the time with their screen brightness at 100%. Here's some recommendations to check:
- Go to Settings -> Display and make sure your brightness isn't at 100%. Personally, I use Power Control Plus (in the market) with a three-stage brightness button: 10%, 40%, 100%. I can just tap the button and cycle between them for whatever lighting situation I'm in.
- Setup some widget buttons or something (again, I use Power Control Plus) to make it easy to turn WiFi and Bluetooth on and off. Bluetooth especially is a big power drain. You can also go to Settings -> Wireless and Networks -> WiFi Settings and uncheck the "Notify" option. This will cause your phone to *constantly* (like every 5 minutes) scan for open WiFi connections, which can be a huge battery drain.
- Go to Settings -> Wireless and Networks -> WiFi Settings and hit the "Menu" button at the bottom of the phone (the one down there by Home, Back, Search). This will bring up a menu at the bottom of the screen with an "Advanced" button. Tap that you'll get to a few more options. One of these is "WiFi sleep policy". By default, it's set to turn off the WiFi whenever the screen is turned off. Sounds like a great idea until you realize that the 3G radio just clicks back on as soon as the WiFi radio turns off and drains even more power than the WiFi does. I have mine set to "Never". If you do this *make sure* you manually turn your WiFi off when your not using it, like I mentioned above, but it will get you the best battery life while you've got WiFi access.
- Check any Facebook/Twitter/Flikr/etc apps for how often their sync'ing. I used to have a Moto Android device and ran the widget that displays all your news feeds (can't remember what it's called to save my life) but I think the default update was like every 15 minutes. What I started doing was running TweetDeck, since I can setup different "channels" to display different info. I have the main feed that is set to only update once a day, and another channel that shows replies to things that I've commented on that updates every 2 hours, and a third channel which is specifically private messages sent to me, which updates every 15 minutes. Admittedly, I'm not the kind of person who's on Facebook all the time, but with this setup, I can just manually tell it to refresh all the channels whenever I'm actually in the mood to read the messages and not have it killing the battery all day unnecessarily.
- Don't use task killers that automatically kill off all your apps. Most of the time, these apps will just have to relaunch, which uses more power than they were using just sitting there.
- If you're using a weather widget, make sure the refresh is a bit lower. These seem to kill the battery quick. I'm actually running Beautiful Widgets right now and have it set so the weather is updated each time I unlock the phone, but not at any other times. This may *not* be the best solution for everyone but, since I'm at work all day, I may not use the phone for hours at a time. That way I know I'm always looking at the most recent weather data when I wake my phone, but it's not just syncing for no reason while it's in my pocket.
Keeping in mind that I'm rooted and have SetCPU running to throttle my CPU clock when the battery gets low in order to conserve juice, I can get will over 30 hours on a single charge with moderate usage.
Some food for thought, guys. Sorry about the *enormous* post

Good luck!