I made a chart with the release dates of the main Pokemon games.
Judging by this, Pokemon Grey (or whatever else the third version may be called) should be coming out in Japan in about a year.
The duration of time between the release of
Pokemon Diamond/Pearl and Platinum is 1 year, 11 months, 16 days.
The duration of time between the release of
Pokemon Ruby/Sapphire and Emerald is 1 year, 9 months, 26 days.
If the
third Pokemon game follows this order, it should come out sometime in 2012 and be revealed later this year.
Do note that I am using Japanese release dates.