Well, I asked the same question, and here's what I found:
"What is the TM symbol?
The (TM) symbol is for all other trademarks that are not registered, they must not be associated with the R-symbol. Doing so can evoke penalties from the USPTO if you eventually try to register your trademark, or if you try to defend against potential infringers. Instead, use the TM symbol in place of the R-symbol, until your trademark is registered. It provides notice to would-be infringers that you view your mark as a trademark, and that you will defend it against would-be infringers."
