According to the excellent blog The Spam Diaries, clients of Ameritrade may have been targeted by pump-and-dump scams. NANA's entries do confirm this is the second time this type of leakage occurs (here and here). Hacking or (most probably) insider leaks?

About 8 months ago, Ameritrade lost 4m$ (while eTrade lost 18m$) as fraudsters used stolen account credentials of legitimate clients to massively buy penny stocks they handled on the other hand. Good news, some of these bank robbers seem to have been arrested.

Electronic brokerage providers do face a large specter of fraud schemes such as phishing websites, malware, pump&dump... But that is not enough for this company: they also lose backup tapes with 200.000 clients' details.