Blackberries worked on 9/11 in NYC as long as you weren't stuck underground on the subway. (All of the phone lines to NYC were jammed practically all day, so it didn't matter whether you were trying to use your mobile or a landline phone.) The explanation I was given -- similar to dogplasma's explanation -- was that the e-mails were broken up into packets and so didn't get tied up the way "regular" phone calls were.

So, I bought some RIMM stock the minute the stock markets opened again, and made a nice bit of money, but it turns out I sold that stock too #$%* soon!