In a nutshell, it went like this:
1. Dwolla's marketing claimed that Dwolla payments were irreversable.
2. Because of (1) lots of exchanges accepted Dwolla.
3. People started defrauding Dwolla using stolen bank account information, transferring money to exchanges and running off with the coins.
4. Rather than sucking it up and improving their security measures, Dwolla started secretly debiting the exchanges Dwolla accounts - erasing the fraudulant transactions and clawing the money back from the exchanges.
5. Once the exchanges figured this out, there was quite an uproar. ExchangeBitcoins stopped accepting Dwolla and closed down soon after. Tradehill did the same (and sued Dwolla as I understand it).
Since then it seems like Dwolla has beefed up their verification standards so this is less of an issue now, but lots of people are still (justifiably) butthurt about it.
Ahh. OK. Thanks for the explanation. I'd be pissed to if I ended up getting screwed because of Dwolla too, even if it seems like they have fixed things.