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.
If you think this is somehow wrong and will make the I/O slower, I suggest you read this...