I think the 140-character constraint is a huge part of Twitter's identity and success. It forces concision. It's one of those constraints that breeds creativity.
And remember that the mobile Internet in the US is pretty good. In much of the rest of the world (the places where people have mobile phones but don't have computers) mobile Internet is still slow and poky. The switch-over from SMS to Internet hasn't happened even in the US yet, so it will be a long while before it becomes a good idea for Twitter to drop SMS, on the order of 5-10 years.
This pegs 'txt' as pretty darn low (3%): http://tweetstats.com/twitter_stats
Also months stats from Jan: http://dcortesi.com/2009/02/19/the-real-top-20-twitter-appli...
I stand by the value of SMS as a driver of valuable content, and in the developing world.