I am using the default cart.js file in my checkout which is included with CT2.5 This provides a copy billing info to shipping function which always seemed to work well, but since upgrading to CT2.5 no longer copies the billing country to shipping country, all other fields copy fine.
Just ran across your question now. Perhaps this answer will help others who are looking.
This link goes to an archived Cartthrob forum question/answer from April 2012 which solved the problem for me:
Are you asking about the example cart/checkout template? There looks to be an error that is keeping the selects from updating in the shipping fields. If you’re using the default cart, open the cart/_header template and change $(“input to $(”:input in the code that updates the shipping fields.
Edited to add:
As of JQuery 1.7, you should use .on() instead of .live() so you should check that out as well. Here's a brief explanation: https://stackoverflow.com/questions/18395786/live-not-working-with-change-function
Hope this helps.