[mootothemax]

With bitcoin, things are not at all reversible.

True of plenty of payment methods, e.g. CHAPS in the UK:

Unlike other forms of payment such as cheques, CHAPS payments are irrevocable

http://www.ukpayments.org.uk/payment_options/chaps/

[ubernostrum]

I think a better word is "traceable". Presumably with CHAPS one can find out who the money went to, and other mechanisms can come into play to get it back.

Bitcoin, though, is supposed to herald the awesome future of totally anonymous, totally untraceable transactions in which once your money's gone, you're fucked.