Please use the following advice with care and remember that it is illegal to crack WEP passwords without the owner's express permission.
Okay, so I've got my Eee PC 900 which is currently kitted out with Ubuntu 9.10.
And then I added Aircrack-ng:
sudo apt-get install aircrack-ng
And it was good.
But it got better when I downloaded a GUI for it from here, extracted it to my home folder and...
...now I have no need of Backtrack!
I should also point out that this worked on my brother's 701 4GB and my other brother's 1005HA.
Um, and that's it. Short and to the point.