Good news for Zune 30 users! Following the Nov 13 firmware upgrade, a lot of people are noting an improvement in battery life.  To achieve this, we improved the Zune 30 codecs so they are more power-efficient, hence the better battery life.

That said, we have seen some reports of a separate battery issue related to the upgrade. This problem occurs when users suspend their Zune hard drives by holding down the play/pause button – in some cases the battery continues to drain. It seems to be affecting only a small percentage of Zune users at this point, but we’re working on including it in our December update (similar to last year) – stay tuned for details and specifics.

A way to try fixing this problem is available here but I'm not sure if it works in all cases:
Issues with Zune 30 v2 Battery Life?

Related articles (December 2006):
Zune Software Update v1.2
Zune Firmware v1.2 is out