There are over three billion cell phone accounts worldwide. Realizing some people have more than one account, some are lapsed and never got shut down; that's still that's creeping up on half the world's population.

With that in mind, guess what advertising channel is growing like gangbusters?

Yup! MediaPost just published a new study projecting mobile advertising worldwide to mushroom from $2.7 billion last year to $19.1 billion by 2012. Here in the United States, companies spent some $878 million in mobile ad campaigns last year. MediaPost is predicting that figure will jump to $19 million by 2012 (smaller numbers, but a more dramatic percentage in growth).

The vast majority of mobile ad campaigns are through text messaging. Search and display ads on the little screen are showing up, but until 3G phones hit critical mass text ads will remain king.

Advertising through text messaging is something for small to midsize businesses to consider. It's relatively cheap and gets a higher response rate (12%) than desktop ads on the Internet.

However, before you consider taking the plunge consider the following:

- Currently, 72% of worldwide consumers that respond to text ads are under the age of 35. If this isn't your customer base, you can click off right here.

- This is optimal for repeat customers, not generating new leads. Most text ad services prevent companies from spamming through text messaging (rightly so, I say!). Customers have to opt in for the messages. For a willing customer base, it's a great way to announce promotions, specials, coupons, new products and services.

- Make sure your customers understand they pay the freight on text messaging charges as structured in their mobile plan. If they don't have a flat rate plan, they may opt out the first time they get their next cell phone bill. For that reason, be conservative in the number of text ads you send out in a given period.

- Write the ads carefully. There's plenty of resources on the web to guide you in how to write a good text ad. Rule number one: keep it short and sweet. Two lines, tops!

- Offer something they can't get through other marketing channels in your business.