Gmail rocks so much, I don't really care if they are reading my mail. ;-)
Seriously, though - it is an automated system, though it is possible that someone could 'break in' and read your mail - so I'd always be aware of that possibility no matter what email client you use.
Also, if you use Gmail, do you use the keyboard shortcuts? They *rock*!
http://mail.google.com/support/bin/a...594&topic=1546
Why move your hands back and forth between the mouse and keyboard?? Using the shortcuts is much more efficient.