This info from the Sprint forums:
CONFIRMED: Fix for HTC Hero "Dialer"...
THIS IS THE FIX!!!!
The Messages app is keeping the phone from sleeping, and the fix is to use a different app for texting:
Go download ChompSMS for free from Marketplace. Use that instead, it looks just like iPhone SMS and works great. I've tried sending/receiving a bunch of texts and Dialer is behaving!!! I LOVE ANDROID. No other mobile OS could you just download a different SMS client!
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More of a work around than a fix but you are 100% correct! Just tested and confirmed on my phone too. I don't really love the client heh. But My phone is sleeping properly after having sent a text through comp instead of through the original sms client. And I agree completely, no other phone would let you just do that.
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When you start up the phone, make sure you are the one putting it to sleep and it's not auto sleep (screen dims). Then it sleeps.
Finally when sending/receiving texts use either ChompSMS or another SMS client other than the built-in Messages app and you will be flying high!!
Make sure you reboot (poweroff/on) your phone before you start using ChompSMS or another SMS app exclusively so any issue caused by the Messages app has been cleared.
Not only is the battery drain gone for me my phone is charging twice as fast!
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Go to Messages (The app) > Settings > Notifications. You can turn it off there. I had been using chompsms since I got the phone, knew about it before I got it, and it's a great app.