Google Android 2.3 User Manual page 144

Gmail
When reading a message or when its tab is selected, you can press Menu
touch Mark unread to return a read message to the unread state—for example, to
remind yourself to read it again later.
AUG-2.3-103
Touch the tab that indicates the number
of unread messages to view their
summary tabs.
Touch the tab of a message
you've already read to open it,
so you can read it again.
Android User's Guide
144
and