Shouldn't colon be reserved?

Hi all!

Pardon my ignorance but I'm rather new to XML. What I was wondering is
that all the characters that play a special role in PDATA, like single
and double quotes, angle brackets and the ampersand are reserved
characters. Then why is it that colon, which is used to specify xmlns
prefixes not reserved? Is it for the fear of breaking existing XML
apps or for some other reason?

-- 
Tahir Hashmi