As of Android 2.2 (Froyo) and iOS 3.0, developers and marketers are able to completely bypass the SMS network to send real-time messages to devices at no cost. These so-called "push notifications" (or  C2DM / Cloud 2 Device Messaging on Android) represent an entirely new class of mobile messaging and  pose a significant threat to traditional SMS technology.

Depending on the platform, developers can typically send push notifications to users once they have  installed the developer's smartphone app on their phone. In that sense, installation of the app is tantamount to the "double optin" familiar to sms marketers. When users remove the app, the developer can't send them further push notifications -- similar to an sms opt-out.

The push notifications appear differently on the phone depending on the platform and model, but in general they appear similar to SMS on the handset -- as a popup or tray notification.  Google has taken this one step further with their C2DM system on Android (2.2 and later), enabling developers to push much more than a simple text notification. C2DM enables developers to remotely "wake up" an inactive app, which can  then display any rich media on the screen even without prompting the user.  An advertiser's wet dream? Yes...be on the look out for a next generation of mobile ad networks / ad units which based on  C2DM ad inventory rather than in-app ads.

The question arises of why would any developer or marketer use SMS anymore, if  they can send free, real-time, and in many cases rich media messages to the device using push notifications? In the near-term, the SMS  ecosystem has little to fear due to low penetration rates for push notifications  -- smartphones are still under 30% of the total handset market, and 50% of new phone sales are still feature phones which don't support push notifications.  It will take a very long time for push notification support to achieve the kind of ubiquitous coverage which SMS enjoys.

Within 3 - 5 years, push notifications could very well overtake SMS to become the primary mobile messaging technology  for marketers and developers. Since the  technology is so new, there are very few good messaging and targeting platforms for push notifications yet. Our company is building one, and I'm sure many others are as well. I expect push notification platforms and ad networks to be a strong theme in mobile over the coming years.