Just one small problem, since last summer trailers have been running for a joint project between Tim Burton and Timur Bekmambetov, (director of Nightwatch and Wanted), called 9. This one revolves around some sort of dolls roaming around a post-apocalyptic wasteland.
There's nothing to stop two people from using the same name for their movie. David Cronenberg and Paul Haggis both released movies called Crash, (one was about sex and car wrecks, the other about the way peoples lives intersect with others). Then there's Gladiator, a title shared by the Russell Crowe Roman epic and a Cuba Gooding Jr. film that people always think is going to be the Russell Crowe movie when they see it in their onscreen cable guide.
However, these movies were released years apart from their fellow namesakes so when it comes to promoting it, there isn't much confusion. On the other hand, 9 and Nine are both being released within months of each other.
So is anybody going to budge? Usually somebody does. In 1998, Alex Proyas' film Dark City was forced to go through several name changes according to the bottom of this article. First, producers of the film Mad City complained it sounded too similar to their movie, so it was changed to Dark World, then Spielberg told them to change it because it sounded too similar to Lost World, so it got changed to Dark Empire, until somebody (George Lucas, supposedly) told them to change it. Of course by this time Mad City bombed and was forgotten, so the name went back to the first choice.
This is also the case with new James Cameron movie Avatar, the cinematic piece of vaporware that he's been talking about making for years and is now apparently actually getting underway. He apparently told the producers of the film adaptation of Avatar: The Last Airbender to simply shorten the title down to The Last Airbender to avoid confusion, even though the latter is a popular Nickelodeon series that debuted in 2005... long before Cameron's movie was due to hit screens.
As for the two "Nines," it isn't clear who'd have more clout. The Rob Marshall musical is based on an Broadway musical (albeit a somewhat obscure one) so for marketing purposes it only makes sense to keep the original name. However, it's also possible this film might be aimed more towards the art house crowd and therefore not get a wide release.
There's no question that Tim Burton's movie is getting a wide release, and as I said before it's been promoted since last year. This one was based on a short film by Shane Acker (who is also directing the full length remake) so it too has a claim to it's title. On the other hand, it hasn't necessarily been ingrained in the public consciousness that the Burton-produced movie is actually called 9 since the trailer flashes the title briefly before showing it's release date, 9-9-09. So it's possible that it can undergo a name change without confusing audiences. After all, the original trailer for Cloverfield never gave the title, just the release date of 1-18-08 (I know, it's lame that I can actually remember that).
Who knows? Maybe neither will budge, but if not I can see a lot of confusion happening here. It's easy to see somebody going to the ticket booth and asking to see "nine," and instead of getting the bleak CGI film they expected, they find themselves in a colorful live action musical. We won't know until they come out.