Hi, Doug from YouMail here.
Just noticed my words and thought I'd give an update - although not much good news.
I basically spent the last 2 days completely reworking the YouMail app's back end code so that anything and everything that could possibly be shut down when it isn't actually doing anything is indeed shut down.
When it came time to test it out -- 100% awake time again (although I stand by my earlier comments that "its not just us").
If I run the same app on a HTC G1 the phone will last a good 2 days (maybe more) without battery issues. If I uninstall the app the difference is minor.
This information, added to all the discussion on the Sprint forums about how the HTC "extras" that this phone comes bundled with have caused the same problem leads me to think that there is something in the Hero that's not behaving properly.
I see other comments like "It does the same on my <insert phone model here>" - well, we don't have unlimited resources, so if you can put the small amount of effort in to dial the 'dump' code mentioned above (*#*#963867#*#*) then maybe we can start to find a pattern here (maybe it's all the Android 1.5 based devices for example).
I'd also like to thank crunchybutternut for all of his assistance and time in trying to sort this issue out.