Preview at WWDC likely in June, followed by September launch.
Video Shows Benefits of 2GB RAM in iPhone 6s
iDownloadBlog recorded this video showing the difference between the iPhone 6 and iPhone 6s after loading several websites in Safari:
The iPhone 6s is able to keep more websites active in memory without requiring a reload when returning to the tab. The additional RAM should also allow more apps to remain active in memory without relaunching.
The iPhone 6s and 6s Plus just launched on Friday.