It happened just a few moments ago:
My daily tweet limit has been exceeded. At first I was a bit shocked - what? Twitter is limiting how much I can use their service?
Then I realized, ah, this is about pushing subscriptions for “unlimited tweeting*” no doubt (*Subject to terms and conditions of course).
While it sucks, I kind of like the idea, and it’s going to push me to subscribe. I think twitter is having a hugely positive impact on free speech currently and I hope this trend continues.