Discussed at considerable length a month ago:

http://news.ycombinator.com/item?id=2895474

At that time I said this:

    It feels to me that all he's done is show that the
    "instantly" in the usual definition of markets being
    "efficient" is a nonsense. Prices must take time to
    compute, you can't know the correct price instantly
    even with access to all the past information.

    He proves that, but it doesn't seem that surprising
    to me.

http://news.ycombinator.com/item?id=2895531

That was expanded on a great deal, and other observations made.

There was also an earlier version of the paper submitted some 19 months ago, also with a great deal of discussion:

http://news.ycombinator.com/item?id=1144548