I think it may be on by default. Either way, it works for me, just not as a DirectPath I/O device, which is probably better because it seems to only enable passthrough on the port when I plug something in. Ultimately, this actually looks to be easier and more elegant.
After boot:
2013-01-11T15:44:59.719Z cpu20:8790)Loading module usb-storage ...
2013-01-11T15:44:59.720Z cpu20:8790)Elf: 1852: module usb-storage has license GPL
2013-01-11T15:44:59.722Z cpu20:8790)module heap vmklnx_usb_storage: creation succeeded. id = 0x41000f600000
2013-01-11T15:44:59.722Z cpu20:8790)<6>Initializing USB Mass Storage driver...
2013-01-11T15:44:59.722Z cpu20:8790)<6>usb 2-1.5: Vendor: 0x8564, Product: 0x1000, Revision: 0x0100
2013-01-11T15:44:59.722Z cpu20:8790)<6>usb 2-1.5: Interface Subclass: 0x06, Protocol: 0x50
2013-01-11T15:45:00.121Z cpu20:8790)WARNING: LinScsiLLD: scsi_add_host:601:vmkAdapter (usb-storage) sgMaxEntries rounded to 255. Reported size was 255
2013-01-11T15:45:00.122Z cpu20:8790)<6>usb-storage 2-1.5:1.0: interface is claimed by usb-storage
2013-01-11T15:45:00.122Z cpu20:8790)<6>usb 2-1.5: device is not available for passthrough
2013-01-11T15:45:00.122Z cpu20:8790)<6>usbcore: registered new interface driver usb-storage
2013-01-11T15:45:00.122Z cpu20:8790)<6>USB Mass Storage support registered.
2013-01-11T15:45:00.122Z cpu20:8790)Mod: 4485: Initialization of usb-storage succeeded with module ID 56.
2013-01-11T15:45:00.122Z cpu20:8790)usb-storage loaded successfully.
2013-01-11T15:45:01.126Z cpu14:8791)usb-storage: detected SCSI revision number 2 on vmhba32
2013-01-11T15:48:42.293Z cpu12:8791)usb-storage: detected SCSI revision number 2 on vmhba32
2013-01-11T15:48:51.781Z cpu16:9033)<6>usb passthrough enabled; all eligible devices will be unclaimed by kernel drivers except for ESXi boot device vmhba32
After plugging in a UFD:
2013-01-11T16:50:15.609Z cpu19:8728)<6>usb 2-1.3: new high speed USB device using ehci_hcd and address 4
2013-01-11T16:50:15.729Z cpu19:8728)<6>usb 2-1.3: New USB device found, idVendor=1b1c, idProduct=1a90
2013-01-11T16:50:15.729Z cpu19:8728)<6>usb 2-1.3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
2013-01-11T16:50:15.729Z cpu19:8728)<6>usb 2-1.3: Product: VoyagerGT
2013-01-11T16:50:15.729Z cpu19:8728)<6>usb 2-1.3: Manufacturer: Corsair
2013-01-11T16:50:15.729Z cpu19:8728)<6>usb 2-1.3: SerialNumber: AA04012700009030
2013-01-11T16:50:15.730Z cpu19:8728)<6>usb 2-1.3: Vendor: 0x1b1c, Product: 0x1a90, Revision: 0x1100
2013-01-11T16:50:15.730Z cpu19:8728)<6>usb 2-1.3: Interface Subclass: 0x06, Protocol: 0x50
2013-01-11T16:50:15.731Z cpu19:8728)WARNING: LinScsiLLD: scsi_add_host:601:vmkAdapter (usb-storage) sgMaxEntries rounded to 255. Reported size was 255
2013-01-11T16:50:15.732Z cpu19:8728)<6>usb-storage 2-1.3:1.0: interface is claimed by usb-storage
2013-01-11T16:50:15.732Z cpu19:8728)<6>usb 2-1.3: device is not available for passthrough
2013-01-11T16:50:15.732Z cpu19:8728)<6>usb 2-1.3: usbfs: registered usb0204
2013-01-11T16:50:17.788Z cpu16:12849)usb-storage: detected SCSI revision number 0 on vmhba39
2013-01-11T16:50:17.788Z cpu16:12849)usb-storage: patching inquiry data to change SCSI revision number from 0 to 2 on vmhba39
2013-01-11T16:50:17.788Z cpu16:12849)usb-storage: patching inquiry data to clear TPGS (3) on vmhba39
2013-01-11T16:50:17.788Z cpu16:12849)usb-storage: setting peripheral qualifier to 'not connected' on vmhba39
2013-01-11T16:50:17.788Z cpu6:8708)<6>usb-storage 2-1.3:1.0: unclaiming vmhba39
2013-01-11T16:50:17.789Z cpu6:8708)<6>usb 2-1.3: device is available for passthrough
Then, add a USB Controller to the guest and click ok. Then, add the USB device.
Image may be NSFW.
Clik here to view.
If you think this is somehow wrong and will make the I/O slower, I suggest you read this...