Sad to say, but I recently installed ubuntu 9.10 (karmic koala) on a Fujitsu Siemens Lifebook S4572. I installed the minimal system followed by xubuntu-desktop and gcompris: xfce is supposed to be better for small boxes and gcompris is the whole point.
The result swapped from the moment it booted: gnome-volume-control used 7MB, the network manager used almost 30MB, and so on. I ran top, pressed m to sort by memory, and uninstalled the most RAM-hungry processes using apt-get remove. Most of them were part of gnome.
Replacing gdm with xdm and disabling all the plugins I found helped make the laptop usable. At the moment it allocates 180MB in order to show the login screen, but at least it's possible to play gcompris (which requires 40MB RAM) on a machine with only 256MB.
I don't know why openoffice.org, abiword and so on were there. Two word processors as part of a minimal install? apt-get remove took care of it.
WLAN won't work. The 4572 apparently contains a PCI ID whitelist, so adding a mini-pci card requires BIOS hacking. Even Fujitsu cards don't work. The wired ethernet works though, and occasional network access is good enough for this computer.
Sound output works, except that for some reason, the
channel was set to mute. I didn't see any obvious button to click in
XFCE or with alsamixer. Installing kmix solved this minor problem (at
the cost of some RAM and/or hassle).
The laptop is too slow to play youtube videos. This may be viewed either as a key feature or a severe problem.