I've been looking around to see if I can find anything like the z50
i.t.o size, weight, and battery life, and while the Picturebook is
somewhat novel, the SR is about the same size, but cheaper:

 http://www.sonystyle.com/vaio/sr/index.shtml

Both models have keyboards that are highly inferior to the z50, IMHO.
The Thinkpad X-series is the next obvious choice, but the models I can
find available are all much more expensive (in the $1800-$2200 range).
For a while, one could find lower-end X-series laptops (x20 or x21; I
forget which) for approximately $1000, but I can't find them anymore.
Given the much greater cost, lower battery life, and annoyance of
spinning media, so far I haven't found anything I like better than the
potential of the z50, although I'm still looking. 

chad