I just wanted to share something I think useful for the raspberry. I was looking for NX session on my raspberry, however I found it was extremely difficult (impossible) to use NX Free Server.
After a time, I found a cool tool, called xpra. It is an equivalent to "screen" for X11. You can launch program using X11 forwarding, then disconnect without killing the program, and reconnect after (like running a browser, launching a download, disconnect and reconnect after to check if finished).
The xpra on raspbian repository doesn't work, so I tried the fork from http://xpra.org/.
Because my SD card is only 2GB, I deactivated the x264 and VPX, not adapted to the raspberry anyway (edit values in setup.py, at the end of the file).
After installed, I can control my X applications on the raspberry from an ubuntu 12.04 without any problems. It's also fluent, no lags (I didn't used any compression to let the Arm breath).
Hope it will help someone