Information is not known. Hopefully soon, before Black and White are released, and those event Pokemon come out. If they are going to do one soon, then it will likely be before Spring. America doesn't get the Mew event often, so I think we will have to wait till after all the Black/White event Pokemon come out.
Edit: Actually, with Pokemon Ranch, I think that a Mew event is even more unlikely, seeing as you can just get it there.