Taking a look at Twitter search, it would appear that regular users passwords have been compromised and are having the tweets sent out under their name. This type of scam is common with IM, but hasn’t been seen often yet on Twitter, minus the hacking of celebrity accounts that took place a couple months ago when hackers penetrated Twitter’s customer support system.
At this point, it’s unclear where the adult website was able to obtain Twitter passwords. The scam seems reminiscent of the one we saw on Google Talk a few weeks ago. As usual, now would be a good time to change your password, especially if you have one that's easy to guess.
We’ll update here if we learn more.
Update: A commenter notes that this is the exact same scam that was seen on Facebook several weeks ago. Details here.