I'm not a friend of the EEE Line anymore (too expensive, bad displays), but I'd prefer the Acer Aspire One over the used X40 anytime. It's a bit lighter, has a _much_ brighter LED backlight display. The displays on the X-series Thinkpads are no-gos (except perhaps the current x61 - which is in another league regarding price). Additionally I want a small energy-efficient SSD on these netbooks, not a 4.2k rpm hdd.
I'm really waiting to buy the Aspire One as a small companion to my MBP 15".