Blair wrong about No. 10

You just can't get away from the issue, can you? Only this isn't about Tony Blair and how long he stays where he is. It's about Tim Blair, and is a follow-up of sorts to this post (which, by coincidence, also originated in Oz). The beef here is that Tim reproduces (from NRO) a short English test, and question 10 of the test is as follows:

10) "Every person is entitled to their own opinion." What's the error in that sentence?

I say there ain't no error. In 2005 'their' is perfectly OK in this context, even if it wasn't always.