I assume this is only where you follow someone.
I have one of my client's tweets posted on his website via Twitter's widget.... No following. Are we going to see ads in this stream? I assume not, but it's unclear.
Twitter certainly has the right to do this, but I think this is going to have people taking a hard look at what has been a fun service. This now appears to be "sign up for spam" to me. Maybe not, but I don't think this is like the annoying ads next to my Hotmail messages from Hotmail. Good luck to them with this.