See lots of other users having problems with the Citrix receiver on 64-bit Natty, so here goes:

Because we are on 64-bit Linux, we only have 64-bit libraries.
Install the 32-bit libraries with:
Code:
$ sudo apt-get install libxaw7 libmotif4 ia32-libs
ICA Client needs the OpenMotif 2.3 libraries (often called OpenMotif version 3, or libmotif4).
Go to the Ubuntu package search: http://packages.ubuntu.com/
Under the header 'Search package directories', search for libmotif4.
Choose the libmotif4 package from natty, version at time of writing: 2.3.3-5ubuntu1, and download the i386 package.
Extract the package:
Code:
$ mkdir libmotif4
$ dpkg -x libmotif4_2.3.3-5ubuntu1_i386.deb libmotif4
Now copy the libraries to /usr/lib32 (make sure they don't overwrite existing files):
Code:
$ cd libmotif4/usr/lib
$ cp -ivR * /usr/lib32
This should install libXm.so.4 in the correct directory, which is needed by the ICA Client.

Go to the Citrix download page: http://www.citrix.com/english/ss/downloads/
Click in the 'Featured Downloads' box on the right on the Linux Clients link.
Download the x86 client tarball. (Note: as of 25 Aug 2011 Citrix supplies a .deb as well as a .rpm and tarball. While I was able to install the .deb, I could not subsequently run the Citrix Receiver it installed. However I was able to run the Citrix Receiver successfully after installing the tarball.)

Download the linux client tarball to some temp dir, e.g. /tmp/citrix
Extract the tarball.
From the download directory, run the text-mode script:
Code:
$ sudo ./setupwfc
Take the option to "Install Citrix Receiver", then install to the default directory: /usr/lib/ICAClient

Check that the executable /usr/lib/ICAClient/wfcmgr was created (and also Applications>Internet>Citrix Receiver, though that's just a link).

Check that the executable /usr/lib/ICAClient/wfcmgr has needed libraries:
Code:
$ CLIENT_EXEC="/usr/lib/ICAClient/wfcmgr"
$ ldd ${CLIENT_EXEC}
You should get results like:
Code:
	linux-gate.so.1 =>  (0xf7718000)
	libXm.so.4 => /usr/lib32/libXm.so.4 (0xf74a6000)
	libXp.so.6 => /usr/lib32/libXp.so.6 (0xf749d000)
	libXpm.so.4 => /usr/lib32/libXpm.so.4 (0xf748c000)
	libSM.so.6 => /usr/lib32/libSM.so.6 (0xf7484000)
	libICE.so.6 => /usr/lib32/libICE.so.6 (0xf746c000)
	libXmu.so.6 => /usr/lib32/libXmu.so.6 (0xf7456000)
	libXinerama.so.1 => /usr/lib32/libXinerama.so.1 (0xf7451000)
	libdl.so.2 => /lib32/libdl.so.2 (0xf744d000)
	libpthread.so.0 => /lib32/libpthread.so.0 (0xf7434000)
	libc.so.6 => /lib32/libc.so.6 (0xf72d7000)
	libXt.so.6 => /usr/lib32/libXt.so.6 (0xf7285000)
	libX11.so.6 => /usr/lib32/libX11.so.6 (0xf716a000)
	libXext.so.6 => /usr/lib32/libXext.so.6 (0xf715a000)
	libXft.so.2 => /usr/lib32/libXft.so.2 (0xf7146000)
	libXau.so.6 => /usr/lib32/libXau.so.6 (0xf7142000)
	libuuid.so.1 => /lib32/libuuid.so.1 (0xf713d000)
	/lib/ld-linux.so.2 (0xf7719000)
	libxcb.so.1 => /usr/lib32/libxcb.so.1 (0xf7124000)
	libfontconfig.so.1 => /usr/lib32/libfontconfig.so.1 (0xf70f4000)
	libfreetype.so.6 => /usr/lib32/libfreetype.so.6 (0xf706e000)
	libXrender.so.1 => /usr/lib32/libXrender.so.1 (0xf7064000)
	libXdmcp.so.6 => /usr/lib32/libXdmcp.so.6 (0xf705e000)
	libz.so.1 => /usr/lib32/libz.so.1 (0xf7049000)
	libexpat.so.1 => /lib32/libexpat.so.1 (0xf701e000)
If you get a "not found" above, you have missed one of the earlier steps of installing 32-bit libs.

If not, try just running /usr/lib/ICAClient/wfcmgr: if that launches the Citrix Receiver, you're done!

For Terminal server, at first launch, Firefox will ask you what to do with the launch.ica file. Choose "Open with" and click the "Browse" button. Browse to /usr/lib/ICAClient/ and click the "wfica" binary.

If you do not want to have to do this every time, tick the box that says "Do this automatically for files like this from now on.", then click "ok".

Terminal server should now work.