Twitter has more than 300m registered accounts (unofficial estimate)

"Twitter just crossed the 300 million account threshold, according to a Twitter-tracking site called Twopcharts.
Twitter is currently adding 9.1 accounts per second. It will reach 301 million in the three hours after this post goes live.
But don't be fooled! Twitter doesn't actually have anywhere near 300 million real users.
In March, we plowed through the numbers and found that Twitter has less than 21 million "active" users."
Source:  Data from Twopcharts, reported by Business Insider, 18th May 2011
Note - here's a link to the earlier research from Business Insider