欧博官网Plain text for the Bitcoin symbol
Update in 2020: The Unicode 10.0 release included the Bitcoin symbol ₿ as U+20BF. For your computer to show the symbol in text, you may need to install a font that includes it. For Linux, one such font is GNU Unifont Glyphs.
To my knowledge there doesn't yet exist a single symbol font representation of the B with two vertical lines, although it seems to be possible to compose it by combining a B and a double vertical stroke overlay: B⃦. However, this doesn't display properly for me.
Some people have been using ฿ instead, however, this is actually the symbol for another currency, the Thai Baht. There has also been some discussion that a B with two vertical lines doesn't distinguish well enough from the Baht symbol.
One could also see Ƀ being used, but its usage doesn't seem to be widespread.
On Bitcoin.it one can find a comprehensive wikipage about the Bitcoin-Symbol. Unfortunately, it has not been updated since August 2013.
Until a proper unicode symbol is introduced, the best option for a disambiguous textual representations for Bitcoin might be one of either of the following.