You know Twitter is viewed as mainstream when its logo is printed onto rail tickets. This was on the back of a ticket I bought in London yesterday, promoting the National Rail Twitter account through which they post service updates. Interesting that National Rail choose to promote a free information service rather than the paid-for iPhone app that brings in revenue (a cool £4.99 per purchase), but hey... that's a different story.
Twitter's mainstream credentials had a boost in the press last week, with Forbes reporting that Twitter now has more than 200 million registered accounts, and more than 110 million Tweets are posted every day. That's a whole load of... um... Tweeps.