OK here it is! If you have more feature requests please do email us. We may consider making a paid version too if there are a lot of features requested. I just tested this out in 4.5.x before I released it and it's pretty simple, but cool!
http://www.bcsengineering.com/store/free-anonymous-customer-registration-for-x-cart.html?MMCF_xfUnAnonymizer
One bonus we never talked about is we have the option to associate old anonymous orders by the same email address to the new user that is created. I think that feature is a great feature to have that I don't think we discussed but it's in there!
Feel free to donate for it too if you like it. It did take more hours than we usually spend on a free mod.
Carrie