The question is, will you accept all this advertising? Is it something that he end-users are going to accept? That's the big question. Obviously we're not talking about 10 second audio adverts inserted into your phone conversations, but using SMS as a [spam like] delivery mechanism could be one option. MMS is still under utilised, but could also make a good delivery system.
No, the majority of the spend is going to be in the browser, and on web based services. The stand-by screens of newer phones (espeically from the network)could easily get a banner, landing and portal pages will be 'mobiled' up with advertising to get the message to you - and with your browser history a very personal watermark on your tastes, AdSense could profile you very well.
Where do you all see mobile advertising going in the next few years?