This was a question Klazien Tilstra, a BA student from the University of Leiden, asked the members of SENSE a few months ago. If you wish to find out the answer, read the summary of her BA thesis on this blog.
Attitudes to the use of singular they are changing: read this blogpost on The Econonist online if you want to know more (with thanks to Johan Rooryck for posting the link on the Leiden MA Linguistics facebook page).
And what do YOU think about singular they?
Bridging the Unbridgeable 
I was surprised to read a few years ago that the US-based New International Version (NIV) of the Bible had adopted gender-neutral language, including the dreaded “singular they”:
http://www.washingtonpost.com/wp-dyn/content/article/2011/03/17/AR2011031703434.html
If it’s good enough for Jesus, isn’t it good enough for the rest of us? :)
Fifteen years or so ago, in a Bible translation into a local Dutch dialect, they’d adopted the Dutch equivalent of periphrastic do (which is pervasive but not part of the standard). It led to a lot of protest at the time as well.
Thanks for the link!