After some testing I managed to create a dualboot setup with ESXi 3.5 and Windows Server 2003.

1. Install ESXi to harddisk 1
2. Install Windows Server 2003 to harddisk 2. Delete the existing partition on harddisk 2 if ESXi has created one.
   On the first reboot during the installation you should get an error “NTLDR is missing”
3. Boot with a live linux cd-rom, eg KNOPPIX
4. (Re-)install syslinux bootloader to boot partition (Hypervisor0)

   sudo syslinux /dev/hda1
   file is read only. overwrite anyway (y/n)? y
   sudo mkdir /mnt/Hypervisor0
   sudo mount /dev/hda1 /mnt/Hypervisor0
   vi /mnt/Hypervisor0/syslinux.cfg
   
   

   original:

   default safeboot.c32
   
   

   modified:

   default menu.c32
   menu title Dual boot
   timeout 100
   label esx
   menu label ESXi 3.5
   COM32 safeboot.c32
   LABEL win
   MENU LABEL MS Windows Server 2003
   COM32 chain.c32
   APPEND boot ntldr=/NTLDR
   
   

5. Copy syslinux modules menu.c32 and chain.c32 to boot partition (Hypervisor0)
   Use locate to find the folder on the live cd that contains the modules, eg locate menu.c32

   cp /usr/lib/syslinux/menu.c32 /mnt/Hypervisor0
   cp /usr/lib/syslinux/chain.c32 /mnt/Hypervisor0
   
   

6. Reboot