For the last month or so I’ve been following the antics of Twitter as they attempt to assert rights to the word "tweet". Based on their completely inconsistent—one might even say schizophrenic approach, I feel justified in using the word "antics".
Until very recently, Twitter didn’t seem to care much about what third-party apps were named. In fact, in a blog post from July 2009, founder Biz Stone said, "Regarding the use of the word Twitter in projects, we are a bit more wary although there are some exceptions here as well. After all, Twitter is the name of our service and our company so the potential for confusion is much higher. When folks ask us about naming their application with "Twitter" we generally respond by suggesting more original branding for their project. This avoids potential confusion down the line."
Note the phrasing: “suggesting more original branding”. What happens if the developers don’t take the suggestion? By allowing so many third party apps to use the name already, they’ve tacitly given permission for others to infringe on their trademark. If they then try to defend their trademark against the new filers, what can they say?