Apache, MySQL, and other common servers and background tasks (daemons) run under the 'nobody' user. From the Wikipedia entry for nobody:
"In many Unix variants, "nobody" is the
conventional name of a user account
which owns no files, is in no
privileged groups, and has no
abilities except those which every
other user has.
It is common to run daemons as nobody,
especially servers, in order to limit
the damage that could be done by a
malicious user who gained control of
them."
You're seeing the 'nobody' user because you're probably running a server or background process on the box. It's nothing to worry about.
