I find it very practical to use an existing UTF-8 character, such as the symbol for Bhat, until there's a unicode character for bitcoin, e.g., using a double-barred-B.

Registration of new unicode characters can be done following this procedure: http://www.unicode.org/pending/proposals.html

Meanwhile, an image with a 'BTC' alt attribute could be used instead, for people who fear the confusion with Bhat.

+1 for BTC as the 3-letter-code for Bitcoins.

My ฿10^-8

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hk