I really enjoyed the Lean UX track at QCon San Francisco 2012. The track opened up with Jeff Gothelf's talk: Better Product definition with lean UX and Design Thinking.
I had a bit of a realization ("ha ha!") when Jeff clarified that requirements should be viewed as assumptions. Then Jeff explained how these assumptions should be proved or disproved in the cheapest way possible. In some cases that's using a minimum viable product (MVP) in other cases mock ups or paper prototypes might do the trick!
Another practical take away was how we could write user stories in the following style:
"We believe that if we implement feature XYZ,
our traders will be able to use it so
they can make more trades per day."
Overall, I thought the presentation made it clear that UX and lean startup concepts and techniques could work well together.
Here are Jeff's slides if you are interested. I am not sure if the slides themselves do justice to Jeff's great presentation so stay tuned for when it becomes available on infoq.com!