Question: I'm trying to run an app from a remote X session (Ubuntu, 9.04) on a handheld device powered by Windows Mobile. I want to do this by running an X-server of some description on the mobile device and using SSH/PocketPuTTY or similar for a tunnel, as you'd do it on a standard PC/Mac.

Can anyone point me in the direction of some X servers for Windows Mobile, if they exist?

Answer: 
I believe that [MI/X](http://www.microimages.com/mix/) will run on Windows Mobile.
At the risk of diverting discussion away from your original question, have you considered simply running a VNC client on your mobile device, and using the built-in Ubuntu desktop sharing tools to connect to the server? 

Generally, VNC degrades much more gracefully when faced with an unreliable network link (a common problem with handhelds), and most clients have support for panning and scaling the virtual desktop to allow a client with a small display to effectively interact with arbitrarily large screen resources.