It is my belief that the terms Global North and Global South still apply. There are many countries like Canada, Sweden and Japan that are obviously Global North, while countries such as Congo, Malawi and Sierra Leone are obviously Global South. Levels of income and life expectancy can you tell that in a second. The labels can still apply to a great deal of countries, and many are moving up to become Global North.
The problem with some nations however is that they are in the process of developing into Global North countries. These countries are adopting capitalist ideas and putting their economies into full gear like China and India. Because they are doing this it is creating a sort of dichotomy within their nations. There is now a Global North and Global South within one nation like China. Some people living in the cities have an excellent standard of living, lots of food, clean water and high life expectancy and they are a part of the Global North. Then there are the working poor and peasants in the countryside. These people are part of the Global South.
So, while you can apply the two terms to whole countries in general, like Canada and Sierra Leone, you have to more flexible when using it to describe developing nations. There can exist both in one country because of the high levels of inequality in some countries that are adopting capitalist practices.