I bought the Webtop travel adapter from Amazon thinking it would work with the 4G Atrix. Sadly I should have read the Amazon comments because the HDMI and USB connectors are round the wrong way.
Not to be put off, I prised the adapter open and was surprised to find two separate PCBs side by side. One for the HDMI (which has no components on it, just tracks connecting the male and female HDMI connectors) the second is for the USB which does have components on it.
I removed the USB PCB and connected it to my Atrix along with the HDMI cable that came packaged with the Webtop adapter. The "Select an application to launch" pop up appeared. No Webtop option, so I chose "Entertainment Centre".
Once this had launched I scrolled to the right 4 places and there is "launch Webtop". Launched webtop, and sure enough it all works using the mouse and keyboard that appear on the Atrix screen.
I did try shorting together pins 4 and 5 on the adapters USB male connector, but it does not force USB OTG, so no USB keyboard unfortunately. All this did was stop the application picker launching at all so I quickly removed it.
It worked for me, cannot guarantee that it will work for everyone. Do mind you fingers if you try prising open the adapter case.