Easy as Cake:
Copy these files from the install CD:
BINMOD.TGZ, CIM.TGZ, IENVIRON.TGZ, LICENSE.TGZ, MBOOT.C32, MENU.C32, OEM.TGZ and VMKERNEL.GZ into your tftpboot Directory.
Insert the normal needed pxelinux.0 File, as well as the pxelinux.cfg Folder which contains following File called "default":
default menu.c32
menu title PXE Boot VMware ESXi
timeout 100
label ESXi3.5U4 Install
kernel mboot.c32
append vmkernel.gz --- binmod.tgz --- ienviron.tgz --- cim.tgz --- oem.tgz --- license.tgz --- install.tgz
ipappend 2
label Hard
menu label Boot from local drive
localboot 0
Have fun!
Taken from: http://www.vm-help.com/esx/esx3i/ESXi_PXE_install.html