According to a recent report from email marketing firm ExactTarget, mobile (SMS in particular) is anticipated to be the fastest growing method used by email marketers to capture opt-in consumer email addresses. ExactTarget's survey found:

Mobile capture was barely on the radar in 2008. While only 2% of marketers used the tactic in 2008, 12% plan to use it in 2009. This represents a six fold increase in adoption, making mobile capture the fastest growing list growth tactic in relative terms.

Here are the top tactics used in 2008:

Here's more forward-looking survey information:

The report promotes the immediacy of SMS as an effective way to gain consumer opt-in to receive marketing materials:

Mobile capture lets consumers text their email address to a shortcode to register or receive information via email. Because mobile phones are close at hand, this method decreases the proximity barrier associated with display and print advertising that encourages consumers to subscribe to online communications. In the past, consumers had to recall a promotion and the associated URL to register—now they can register immediately.