Smart phones are everywhere. Adults and children alike can't put them down, using their crackberries and iPhones for everything from playing games to scanning business docs.
Something else smart phone owners use them for is social media. More specifically, Twitter. A study done by Compete.com, a web traffic analytics service, measured how consumers are using social media, and found that 43% of Twitter users access the service through a mobile device. Also, when it comes to engaging with brands, Twitter is the medium of choice, beating out Facebook and LinkedIn.
The study also found that 56% of Twitter users who follow a brand say they are "more likely" to buy their products, and 84% of Twitter users follow brands for updates on new products. Facebook and LinkedIn are important too, of course, but with all the smart phone addicts out there tweeting their hearts out, Twitter may be the dominant source of marketing to your customers.
If this data doesn't demonstrate the importance of using Twitter to interact with your customers, I don't know what will!