I was trying to install VMware esxi 3.5 on a Dell PowerVault 750N (sold as a nas box).  The box doesn’t have a CD drive so I had to get the esxi install on another way.  I found a couple how-to’s about booting from a usb thumb drive that used a different kernel, but by changing a few bits I was able to boot from a USB thumb drive directly to the Esxi install.

Items Used:
1. 8GB USB 2.0 thumb drive  ***Revised I had to use a 512 stick to get it working on 1 server
2. VMware-VMvisor-InstallerCD-3.5.0xxxx.i386.iso (extract iso to USB drive)
3. Syslinux-3.72 (unzip to temp location)

Procedure:
1. Format USB drive with FAT32 (I needed a 512MB USB drive formated FAT to get it to work on 1 server)
2. Extract VMware iso to USB drive
3. Extract Syslinux onto C: drive
4. Navigate to syslinux-3.72\win32 and run the following command
syslinux.exe -s -m -f -a e: (e: was my usb drive)
5. This will create a file called ldlinux.sys (hidden file so you need to view hidden to see it)
6. Navigate to the USB drive and rename isolinux.cfg to syslinux.cfg. You do not need to change anything in this file. The config file boots the mboot.c32 kernel that is included with the esxi install.
7. Make sure your server is set to boot from USB and you are good to go.

Other Helpful Links:

Boot ESXi from USB Stick – (not same as installing from USB)
Boot ESX from USB drive