The links I posted in the shoutbox...
http://www.wsu.edu:8080/~brians/errors/they.html
http://www.proz.com/topic/23556
http://www.crossmyt.com/hc/linghebr/austheir.html
I think "their" is a good substitute for "his/her," and to me (American, English is my native language) it seems perfectly acceptable.