One little note: UNetbootin won't see your USB drive if it's formatted as Mac OS Extended (Journaled) with GUID Partition Table. That's easy to fix with Disk Utility:
- select your USB drive from the list of devices on the left
- click on the "Erase" tab on the right
- choose "MS-DOS (FAT)" format
- click "Erase" button
Now on to create the LiveUSB VMware vSphere 5 installer. Open UNetbotin, enter your Mac administrator password and then:
- select "Diskimage" option
- browse to and select your previously downloaded VMware ESXi 5 Installer iso
- make sure your installing onto correct USB drive
- click "OK" and wait for wizard to complete
When wizard asks whether you want to overwrite
menu.c32 file, answer "No". This will allow you to boot directly into ESXi installer and bypass Syslinux default blue boot menu.
Finish, eject and you're ready to install your sparkling new Hypervisor.
Update: as you can see in comments below, it seems these steps are not quite sufficient to boot ESXi installer. I also had to enable UEFI boot option in BIOS, while Raul added a kickstart configuration.