ACH transfers are reversible and a few bitcoin exchanges have been bitten by them since most have/do take dwolla, which is a nice service for ACH transfers.
Bank wire transfers, though, are unable to be reversed. Its a pity that most US banks charge for them.
Yeah, the US banking system is pretty pitiful in general. We (Balanced) are trying to make it a little bit better by making ACH faster and easier to integrate, but we can't do much about reversals. [shameless plug] Example, checkout our ACH payouts feature: blog.balancedpayments.com/announcing-balanced-payouts/
Bank wire transfers, though, are unable to be reversed. Its a pity that most US banks charge for them.