There is a nice online writeup of connecting a lapdock to many devices, including a RasPi at:http://www.robpol86.com/index.php/Atrix ... Other_Uses
The author includes a list of what cables and adapters he used for each setup, and from where he got them. There are other references online about using a lapdock with a RasPi. I suggest you do a web search with keywords "Motorola lapdock".
There is more than one combination of cables and adapters that will suffice. Of all the combinations I saw, no one had been able to avoid ordering from China/Hong Kong. Anything from there takes a long time to ship (maybe 3 weeks), but they are fairly cheap.
The cables mentioned earlier in this thread are insufficient by themselves. Note that the HDMI cable has male micro connectors on both ends. You need to end up with a female micro HDMI connected to a male full-size HDMI. The other cable has one appropriate end, the female micro USB. The other end needs to be a male full-size USB, so an adapter would need to be aded. All of the solutions I saw online required a combination cable and an adapter for both the HDMI and the USB; so a minimum of four pieces. I'm still waiting for my China order!