Based on the idea of TiagoTiago:

http://img593.imageshack.us/img593/180/btclogosent0.pnghttp://img215.imageshack.us/img215/6297/btclogosent1.png http://img219.imageshack.us/img219/8246/btclogosent2.png

Available as SVG. (Needs some more work.)

I like the one on the left but maybe it looks more like a logo than a currency symbol. What do you think?

EDIT: Also, Ⓑ is pretty convenient IMO.
EDIT: Uploaded background transparent versions. I'll work on the SVG on demand.