Citrix is a way of remotely connecting into your work and being able to run applications as if you were in the office. A Citrix client would be a great enhancement for business use of the N900, the high res screen and keyboard make it an ideal mobile client device.
There are currently several mobile clients for other devices, and there has been reports in the forum that the arm build is installable, but it is not really usable. Unfortunately I believe it is closed source, so it is difficult for the community to find a solution without help.
Talk thread: http://talk.maemo.org/showthread.php?t=49502
