Preview at WWDC likely in June, followed by September launch.
AT&T Alone Sees 200,000 iPhone 4S Preorders in First 12 Hours, Their Most Successful iPhone Launch Ever
Reuters reports that AT&T is seeing "extraordinary demand" for the new iPhone 4S which became available for preorder at around 12:01 am Pacific time on Friday, October 7th. AT&T reveals that they have seen over 200,000 preorders for the iPhone 4S in the first 12 hours of sale.
For reference, last year, Apple announced that they saw a record-breaking 600,000 iPhone 4 pre-orders in the first day across all carriers in US, France, Germany, Japan and the UK. So, 2010's 600,000 number included all of Apple's launch carriers and over the first full day of sales.
This year, AT&T alone saw over 200,000 pre-orders in half the time. Unlike last year, the new iPhone 4S is now also available on Verizon and Sprint in the U.S, which will account for many other U.S. preorders. Despite being split across three carriers, AT&T says that this is "the most successful iPhone launch we’ve ever had".
Similarly, Apple also offered iPhone 4S preorders in several other countries outside the U.S. too (Australia, Canada, France, Germany, Japan and the UK). So, it seems certain that the iPhone 4S will also be Apple's most successful phone launch ever.
Update: Sprint's website shows the 16GB iPhone 4S as sold out.