I had to setup ESXi on some brand new Gen 8 HP ProLiant servers that had two hard drive cages and no optical drive. Since I never carry a USB optical drive this was the best way to go.
I've outline a few very simple steps below to make your USB key bootable with ESXi so just boot from the USB drive and you're good to go.
1. Download UNetbootin and run the software. Here are links to the different versions you may need. (Windows – Mac OS X – Linux).
2. Download the VMware vSphere ISO file. Note to use the newest 5.0.0 update 1 as of this writing. -> VMware Download Center.
3. Start the UNetbootin application and choose Diskimage (ISO) and browse to the downloaded ISO file. There are several options of top for many operating systems and applications if you happen you want to make one of those as well.
4. Choose Type: USB Drive and choose the correct USB drive letter that you want the bootable installer to be installed to.
5. Let the program run and you are finished. Make sure you edit the boot sequence in your BIOS (UEFI mode on some systems) to boot from USB instead of CD/DVD-ROM or HDD.
The vSphere 5 documentation can be found online here.