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[myself]

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[enmaku]

There's a post here:
http://forum.bitcoin.org/index.php?topic=369.220
taking a survey about the preferred unicode character for BTC. The current front-runner is ฿ although technically, that's already the symbol for the Thai Baht unit of currency. 2nd place seems to be Ⓑ with ¤ running a distant third.

Of course this is all assuming that twitter supports unicode - never had a reason to test but i'm pretty sure it does... At least it *should*