Went into the Apple Store to have a look.
Device notably smaller and lighter (side margins around reduced a lot, top/bottom seem same). Did a side by side test with my iPad1 and the Air was like lightening in comparison even for simple web page loads, retina screen obviously much sharper too.
16GB and 32GB models 20/30 in stock of each, 64GB and 128GB sold out.
MacRumors (link) has a few stories also benchmark tests showing material speed improvements over iPad4 for processor and graphics
Just need to save a few pennies, managed to resist the buy it now urge as my daughter can but it less student discount !