iPhone 3GS is available in India (my mom bought one a few months ago).
The iPhone is released roughly every year (June 07, July 08, June 09, June 10). Going by that trend, it is plausible that the next iPhone may launch in about 5 months. It's not like Apple hit the ball out of the park with the iPhone 4. Can't confirm, however, whether it is a major overhaul or just a few hardware upgrades (faster processor, more memory, etc.) like it was from 3G -> 3GS.