Many of the arrested were producing the infamous 419-esque emails that promise money and other fantastic loot if you would but pay a small processing fee. A typical scam would involve a fake check being sent to the victim, then said victim cashing it and sending the money back to the scammer. It is interesting that though a lot of the focus is on Nigeria, the majority of the arrests came from the Netherlands:
The crackdown netted 16 arrests in Nigeria, 60 in the Netherlands and one in Canada, said Greg Campbell, U.S. Postal Inspection Service inspector in charge of global security.
That aside, Nigeria is still being targeted as the prime source of these scams, which can make actually finding and prosecuting a scammer all the more difficult. I'm not foolish enough to think scammers can be stopped by arresting everyone you suspect, but perhaps prosecution combined with better education will result in a cleaner Internet.