Should URL be case sensitive?

I noticed that

HTTP://STACKOVERFLOW.COM/QUESTIONS/ASK

and

http://stackoverflow.com/questions/ask

both works fine – actually the previous one is converted to lowercase.

I think that this makes sense for the user.

If I look at Google then this URL works fine:

http://www.google.com/intl/en/about/corporate/index.html  

but this one with “ABOUT” is not working:

http://www.google.com/intl/en/ABOUT/corporate/index.html   

Should the URL be case sensitive?