Free to Play A Viable Business Model

"Lately, it seems like more and more developers are turning to the free-to-play or partially free-to-play model in order to maintain player bases and hopefully gain revenue. Even Bioware’s once-wildly popular MMO Star Wars: The Old Republic will soon be getting the free-to-play treatment, as announced yesterday. Is this tactic viable for struggling MMOs (in other words, all MMOs without Warcraft in their name)?" -Trevor from Leviathyn.com.