So, Black Cat Networks have a bunch of nice dedicated servers and I've been working on various network installs for them. Debian was of course easy; there's a nice netboot.tar.gz that has everything you need (including configs for serial consoles).

Fedora Core was a bit harder; there's mention about network boots and installs, but not a lot of docs. A pxelinux entry like: 

label fedora
	kernel fc4/vmlinuz
	append initrd=fc4/initrd.img ramdisk_size=8192 console=ttyS0,9600n8 serial

does the right thing, with vmlinuz and initrd.img coming from http://download.fedora.redhat.com/pub/fedora/linux/core/4/i386/os/images/pxeboot/. Fedora's a bit annoying in that it doesn't offer you a selection of mirrors to network install from though, so you need to have worked that out for the install.

FreeBSD however, was much harder. Firstly, there aren't any decent docs about it that I could find. Secondly, it really wants to use NFS, even though TFTP will be required anyway for the PXE boot stage.

So, I ended up recompiling the pxeboot loader with LOADER_TFTP_SUPPORT enabled and LOADER_NFS_SUPPORT removed (though it still tries to do NFS mounts it seems). Of course, you can't do this under Linux easily. Nor can you do it on FreeBSD 4.10 it seems (I was recompiling the FreeBSD 6 loader). However you can on FreeBSD 5.4. You need the lib and sys source trees to do so. Or you can have my prebuilt copy, freebsd.0.

pxelinux config looks like: 

label freebsd
	kernel boot/freebsd.0

assuming you put it in a boot subdirectory under your tftpboot root dir. You then need http://ftp.uk.freebsd.org/pub/FreeBSD/releases/i386/ISO-IMAGES/6.0/6.0-RELEASE-i386-bootonly.iso to get the rest of the /boot contents from. Burn it to CD or loopback mount it or work whatever magic does the trick for you to get access to the files in that image. Then copy all of /boot on it to boot in your tftproot. You don't actually need boot*, cdboot, loader or pxeboot, but they're all small files. Then, add the following to loader.conf: 

boot_serial="YES"
console="comconsole"
kernel_options="-h"
vfs.root.mountfrom="ufs:/dev/md0c"

So, now you can type "freebsd" at your pxelinux boot prompt and it'll boot up the installer. Rock. I don't quite understand why I had to include the kernel_options line above - I thought setting boot_serial would do that for me. However, it works. With the only real pain being having to recompile pxeboot and the lack of docs. Which I've now sorted for you. ;)