The trackball lights up as well as a flashing blue? LED next to where you would put your ear, if holding the phone to talk. On the other side is the light sensor if I am not mistaken.
Edit:
Its actually green and amber, depending.
Solid green Battery is fully charged.
(When the device is connected to the AC adapter/computer.)
Solid amber Battery is charging.
Flashing amber Battery is low. (Battery power is below 14%.)
Flashing green You have a pending notification. (For example, new message or
missed call.)
And
Trackball allows you to select or highlight items on the screen. The trackball also flashes when you receive
an incoming call or a notification.
Both taken from here
http://insidesprintnow.files.wordpress.com/2009/09/hero_full.pdf