I was told by some that the Gmail mailboxes can be accessed by searching on google and that would really be an invasion into one's privacy. It is indeed disturbing something of that sort.
A page pointed to by of the links posted by Gabriela speaks directly to this concern:
Will my Gmail messages appear in Google Search results?
I suppose the people who told you Google searches will access Gmail mailboxes must believe that Google is not telling the truth...
Nevertheless, if Google's statement were not true, it would soon become evident.
[Edited at 2005-03-15 14:47]