No, it's a text file. In BSD, and I assume other wise (are we talking Sendmail, here?) you then create the *.db file:
/usr/sbin/makemap hash virtusertable.db < virtusertable
chmod 0640 virtusertable.db
Now, if your system's arranged like mine, you can simply run "make" in /etc/mail and have it do this. However, "make" is a meta-command that will also build configuration files if the source files have been changed, run "newaliases", rebuild the access.db file, etc.