Did IE10 change the definition of window.location.port?

Question

I noticed some JavaScript code that inspects window.location.port was breaking in IE10 Platform Preview 10.0.1008.16421 today.

For instance, in IE10, go to http://www.microsoft.com. Open up the developer tools and type in window.location.port and it returns 80. What happened? Why does IE10 suddenly return the integer 80 when all other IE browsers return the "" (empty string) on the same site? IE10 also returns 80 in all of the downlevel modes.

Compare the result for IE8/9 on http://www.microsoft.com : window.location.port is ""!

Any idea if this is a new behavior for IE10, a logged bug in the platform release, or a has-yet-to-be-reported bug?