This might be a quite old problem, however I couldn't find a solution for myself. I was using Ubuntu 10.10 on my Asus eee pc 1001p until I upgraded to Natty. Now, whenever I connect to a password secured wireless network, the system freezes and needs to be hard rebooted.
It looks like this 'bug' affects the whole eee-family. In this topic on ubuntuforums, the user 'linux_one' states, that an update to the latest daily mainline kernel from the kernel-ppa would solve the issue. Unfortunately, this didn't prove true for me. My system is also up-to-date right now.
I doubt, that the problem lies in the kernel itself, since the wlan-drivers already caused trouble in the past. I've been using the wireless fixes on http://linuxon1001p.blogspot.com so far.
Any solution or hint is appreciated. I'll give additional information if needed.