As internet-connected mobile devices find their way into the hands of more and more consumers, advertisers are increasingly focusing on the mobile channel.

Earlier this year, comScore found that the number of advertisers buying mobile inventory has grown 120% in two years.

Given the rise of mobile, it’s no surprise that some are suggesting mobile could quickly become one of the most important channels for advertisers.

For instance, Razorfish’s Mobile Practice Lead, Paul Gelb, has predicted that mobile will surpass the $130bn/year television advertising market — and soon.

While Gelb’s prediction may be a tad too optimistic, one thing is certain: the amount of money being spent on mobile ads is growing.

According to eMarketer, advertisers will spend more than $1bn this year in the United States on display, search and SMS-based mobile ads, marking the first time spending in the space has topped the billion-dollar mark.

That’s a significant milestone, but if eMarketer’s forecasts prove right, it won’t be long before it’s surpassed. A year ago, eMarketer believed mobile ad spending in the U.S. would hit $2.5bn by 2014, but that number has now been revised upward to nearly $3.4bn. By 2015, it sees mobile advertising as a $4.4bn market.

The dramatic rise of mobile advertising is easy to understand: smart phone penetration is still rising at a rapid pace and a growing number of consumers are using the mobile web on a regular basis.

That makes mobile an increasingly attractive channel for advertisers.

The evolution and mainstreaming of smart phones and the mobile web will also have an impact on how advertisers reach consumers in the channel. According to eMarketer, the majority of mobile ad spend currently goes to messaging-based ads.

But next year, display and search mobile ads will overtake messaging-based ads. By 2015, mobile display and search ads will each be billion-dollar markets on their own.

Obviously, there are plenty of reasons for advertisers to be excited about the opportunities being created by mobile. But mobile is still going to be a challenging channel for them.

As noted before, mobile devices are personal devices and for advertisers to succeed in reaching consumers effectively through them, they need to ensure that their campaigns respect that.