Is it just me, or is it slightly strange that:

1. Twitter is a business built on lifting an artificial constraint from another medium (140-character posts, suspiciously close to the 160-character limit we know and love in SMS);

2. Meanwhile, there's a vast amount of whingeing going on about how it's deeply unfair that Twitter is turning off mobile updates... all because it's constrained by a different aspect of SMS: cost-per-message-sent;

3. This latter aspect of SMS has probably let to a better ecosystem of profitable businesses (i.e. pretty well the whole mobile content industry from ringtones onwards, let along the direct person-to-person messaging revenues) than most Web 2.0 startups will ever hope to;
   

See also Fred Wilsons lovely post on constraints and why businesses ought to embrace them:

"I believe constraints are key to building great web apps. I am not sure about rules that are dictated by the market or government. But the reality of the place we are in is that we have to deal with them. And the best entrepreneurs will figure out how to play these rules to their advantage."