Google ANDROID 2.3.4 - Manual (Page 144)

Gmail 144
AUG-2.3.4-105 Android User’s Guide
When reading a message or when its tab is selected, you can press Menu and
touch Mark unread to return a read message to the unread state—for example, to
remind yourself to read it again later.
Touch the tab of a message
you’ve already read to open it,
so you can read it again.
Touch the tab that indicates the number
of unread messages to view their
summary tabs.