I'm not sure where this came from originally, but this is the standard I've used... from
https://bitcointalk.org/adrotate.php?adinfoThe Bitcoin software, network, and concept is called "Bitcoin" with a capitalized "B". Bitcoin currency units are called "bitcoins" with a lowercase "b" -- this is often abbreviated BTC.
That is also the convention that has been
generally accepted on the StackExchange site. Basically everyone seems to accept it, there was never any debate.