I tried to upgrade my Eee PC 900 to Ubuntu 9.04, but it introduced a serious problem. Apparently, there is a flaw in the Intel i915 driver that Ubuntu packaged with this version of Linux (the kernel). There is more information at this bug report.
I am about to start trying to fix this now, but in the meantime, consider yourself forewarned. From what I have heard, this bug seems to only affect the 900 model, and the newest array.org kernel is not yet available for Jaunty.