Tom Warren has got Windows Phone 8.1 running on his device and has posted a couple of screenshots of it on twitter that show different backgrounds being used on the Windows Phone homescreen.
While the right one really does not blend in with the metro UI, choice is a good thing. Right now, you can only choose between plain white or black. Custom backgrounds will allow users to create really creative homescreens.
Also, if you can change the background in Windows 8, it makes perfect sense to allow users to change it in Windows Phone as well. This will also make the desktop and mobile version of Windows identical, at least skin-wise. via twitter and twitter.