dealers usually don't know s*** about this... and even if they did, they wouldn't tell anyone. (they just want to sell the cars NOW not give anyone a reason to wait a year.) Mazda reps are usually a good source for this type of information... but getting in touch with one can sometimes prove challenging.
Since they're coming out with a new version in 2010 I doubt a MS model will be available any sooner than 2011... hopefully I'm wrong.
Someone mentioned the Miata and 6 as examples of why this car would follow the same path. The difference is... those cars didn't sell while the MS3 is a huge hit. So Mazda would be wise to continue the momentum and make it a mainstay in their lineup (like the WRX) as opposed to just some speciality car they roll out the last couple of years of a model's lifespan.