BOOTP and TFTP offer almost no security whatsoever. They basically provide their information to anybody who asks and solely rely on the assumption that your network is configured to not make the server world-accessable. If you install BOOTP gateways, then this assumption is seriously violated. Also, TFTP server are usually accessible from just about everywhere. You can try to dimish the impact of this problem by blocking BOOTP and TFTP packes from leaving or entering your network segment, but this will never be a completely secure solution.
Thus you should always assume that all of the files that your BOOTP and TFTP server offer are world readable. They must not contain any sensitive data. Also, the TFTP daemon must be configured to only allow access to selected files. Running it in a chroot'd environment might be a very good idea.
The BOOTP protocol is vulnerable against somebody else impersonating as a BOOTP server. While security aware operating systems, prevent non-priviledged users from starting their own BOOTP servers, other operating systems do not allow this. This means, if any of your users can launch an arbitrary program under an insecure operating system on an arbitrary machine connected to your ethernet segment, then they have full control over the BOOTP boot process.