You know it's a matter of how soon, and based on what Apple did today, i wouldn't be surprised if it was within the next couple of weeks in time for the holiday season
1) iPod touch 16 gb is coming out
2) 4 gb iPhone is being discontinued
3) price of 8 gb iPhone is reduced to 399 which is less than the opening price of the 4gb iPhone
All of the above I think adds up to the fact that Apple is clearing the road for a 16gb iPhone which will most likely take up the "premium" price point of 599