Preview at WWDC likely in June, followed by September launch.
Hands-On Video with 'iPad Mini' Physical Mockup
Similar mockups in both white and black have appeared over the past several weeks showing the rumored thin side bezel design that would make the device more comfortable to hold in one hand than a uniform bezel such as that seen on the full-size iPad. The video also offers a good overview of the external features of the device.
Apple is expected to introduce the iPad mini at an October media event once the publicity from the iPhone 5 launch begins to subside.