Yes, I'm believing that this is a firmware issue as well with all these people having the same problem. I have an 80g and I am getting the same as you all. I would charge it to full, and not even play anything on it - and it'll be dead by the end of the day. I hope someone addresses this soon because I am often left now not knowing whether or not the Zune will have power when I get ready for the commute home from work.
I too think that it's not going into "low power standby" mode.