Editor of the Jargon File here. If there is an authority on this question, I'm it.
Never capitalize domain names unless you know for certain they were registered with that exact capitalization. In practice this means: Never, ever capitalize. If these means you have to rewrite a sentence to avoid having a domain name at the beginning, do so.
The more general rule, which explains this one, is: never capitalize any name with a case-sensitive encoding. Other cases this includes are variable names in mathematical formulas and computer programs, and names of programs on operating systems (like Unix) with case-sensitive filenames.
(Some people will reply that domain names are not case-sensitive. In the new Unicode world this is no longer true. And even in older times, interfaces that expected domain names - including embedding in URLs - were often case-sensitive.)
The reason for this rule is that you never want to mislead humans into remembering and using a case variant that a literal-minded computer will not accept.