To access USB Host mode on the Touchpad (to plug in a Keyboard/USB Drive/etc), you need:

1. A microUSB to USB Female cable (USB-OTG cable), available on ebay for ~$3, shipped [search 'USB-OTG cable']
2. A powered USB Hub (a USB Hub with a spot to plug in an external power supply), available on ebay for under $10, shipped, with power supply  [Search 'powered usb hub']

All you need to do is plug the USB Hub into the OTG adapter, and the OTG Adapter into the Touchpad. Power it with a wall adapter, and you'll have USB devices showing up in dmesg in no time!

For example:

[16075.755719] usb 1-1.1: new high speed USB device using msm_hsusb_host and address 3
[16075.775744] get_port_status port=1,portstatus=1503, portchange=10
[16075.868006] usb 1-1.1: New USB device found, idVendor=13fe, idProduct=3323
[16075.868111] usb 1-1.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[16075.883769] usb 1-1.1: Product: STORE N GO
[16075.883821] usb 1-1.1: Manufacturer: Verbatim
[16075.894000] usb 1-1.1: SerialNumber: 070007A315074099E8C5
[16075.922270] scsi1 : usb-storage 1-1.1:1.0
[16075.944010] get_port_status port=1,portstatus=1503, portchange=0
[16076.953338] scsi 1:0:0:0: Direct-Access     Verbatim STORE N GO       5.00 PQ: 0 ANSI: 0 CCS
[16076.993863] sd 1:0:0:0: Attached scsi generic sg0 type 0
[16077.204050] sd 1:0:0:0: [sda] 7802880 512-byte logical blocks: (3.99 GB/3.72 GiB)
[16077.204665] sd 1:0:0:0: [sda] Write Protect is off
[16077.216317] sd 1:0:0:0: [sda] Mode Sense: 23 00 00 00
[16077.216329] sd 1:0:0:0: [sda] Assuming drive cache: write through
[16077.219671] sd 1:0:0:0: [sda] Assuming drive cache: write through
[16077.227274]  sda: sda1
[16077.237134] sd 1:0:0:0: [sda] Assuming drive cache: write through
[16077.237234] sd 1:0:0:0: [sda] Attached SCSI removable disk

To be fair, there is most likely a way to enable the Touchpad to power USB OTG devices itself, which would remove the need for the powered hub.

See also: http://forums.precentral.net/hp-touchpad/296505-functional-usb-host-touchpad.html and http://www.youtube.com/watch?NR=1&v=rXRcvN50-nU, which suggest using a USB Y-cable to inject power, and from which I blatantly stole the idea of using a powered USB hub ;)

So far I have tried a USB thumbdrive and a wireless mouse dongle; the thumbdrive was detected flawlessly, while the mouse's dongle was not.