after accidentally ordering a dell poweredge r510 server without a cdrom i was desperate to install vmware esxi 4, a bit of googling found:
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.