Yes I think this suggestion is in line with how most Android apps seem to behave. The back is more like "go up a level" (including exiting the app) than "go through the history".

I agree wholeheartedly! To have this even as just an option would dramatically increase the usability for a lot, even if not all, users.

I would think that a large enough percentage of users would prefer not to leave an app running in the background until Android decides to close it, that this feature would be well worth implementing. Plus it would seem such a very simple thing to implement that it shouldn't need to be 'under review' for as long as it has!
I have so far tried probably 15-20 task/project management apps and this is the only one that has this behavior.

yes please this is a good idea.

from the email i sent to the team here.

"This is a good app. Have been testing the free trial app. One major thing it can't seem to do is exit the app once it's been opened.
There is no exit options
Pressing the back button does nothing.
Only way to exit is to press the home key. Which leaves the app running in background
I have a Samsung Galaxy gt-i5800
Running Android 2.2 froyo.
To me this is a big issue and if it is fixed I will most likely purchase the full version.
Hope this helps you out"..........

"still it see you do not recommend to shutdown the app in background. that is a shame as it uses extra battery if running. other apps such as calendars and to lists still work and show reminders without running in background. i would liek to see something sorted to properly exit the app. thanks"..........

"having option to easily quit is good. for most apps
1. if i press the home key the app is active in background, or in standby,
2. a. if pressing the back key it will keep going back to each screen that was opened in reversed order, then when at the first screen either it will have exit/quit option or press it once more and it will shut the app down. and b. press the menu button has an exit/quit option which shuts down the app.

I would prefer to see option 2a or/and 2b. to-do-list is the first that does not have these options.

this is just what people expect to see in android apps these days.

1) On most apps you can use the back button to back all the way out to
the device home screen. With the Ultimate to do list you get locked in
the app and have to use the home button to exit.

2) The back button does not follow the hierarchy in the app. Instead
you go back through every screen you touched since you launched the
app. I would expect that once you are at the app home screen you can
not go back any further, and one more back will exit the app. See the
gmail app for an example. Visit several emails from the main gmail
screen and go back. You never get taken back to the emails you viewed
earlier in the session.