Preview at WWDC likely in June, followed by September launch.
iPads Running iOS 6.0 at Apple's Campus Showing Up in Web Logs
Ars Technica server log entries showing iPads running iOS 6.0 visiting from Apple's campus
In support of the notion that work on iOS 6.0 is progressing, Ars Technica reports that browser user agent strings appearing as iPads running iOS 6.0 are indeed showing up in their server logs. While user agent strings are easily faked, Ars Technica filtered its logs to isolate only those entries coming from IPs assigned to Apple's corporate facilities and found a number of visits from devices reporting themselves as iPads running iOS 6.
[W]e began looking at iPad user agents coming from Apple's corporate IP block in Cupertino and discovered that Apple appears to be surfing the Web using iPads running what looks like iOS 6.0. The whole listing shows iPads running iOS 5, iOS 5.0.1 (the current public release), iOS 5.1 (the upcoming release currently available to developers), and iOS 6. The iPads that appear to be running iOS 6 are also using a slightly newer build of WebKit—the older OSes all show WebKit 534.46, while the ones claiming to be iOS 6 show WebKit build 535.8.The report also cites a separate observation of a 2048x1536 screen resolution showing up in Google Analytics. The entries are not specifically linked to the iPad, but there are few other reasonable displays on the market that would be likely to yield such a resolution.
The problem with speculation about these resolution entries representing the iPad 3 is that the new device should still report 1024x768 as its resolution. With the higher pixel density intended to increase sharpness rather than screen real estate, web content would appear far too small on the device if it reported itself at its true 2048x1536 resolution. This situation can be seen with Retina-capable iPhone 4/4S and iPod touch models, which report their resolutions as 480x320 instead of their actual 960x640 resolutions.