In a word, Yes. The DNS system, upon which all domains are built, eg. hotmail.com, is not case sensitive. Additionally all URLs are non-case sensitive, until you get to the file part of it. For instance, Unix & Linux are case sensitive operating systems and as such a web server running on these systems will interpret http://server.domain.com/filename.extension
as being different from http://server.domain.com/FileName.Extension,
but NOT different from http://Server.Domain.Com/filename.extension.
Hope that's a nice 'n comprehensive answer for you.
No, it's not universal. The first person to answer this wrote a correct explanation of this, but I'd just like to add one thing: The domain name of an e-mail address is the part after the @ sign. That's the only part you can be sure to not be case sensitive. The first part may or may not be sensitive.