Are the mid-2012 Macbook Air compatible with older MagSafe adapters? Will the mid-2012 Macbook Air models be compatible with the MagSafe power supply of the previous generations?
Electrically yes, physically not without an adapter.

Only the physical layout of the pins and metal end that connects to the Air is different. However, with the MagSafe to MagSafe 2 Adaptor ($10) lets any original sized MagSafe adapter slim down to work with the new thinner Mac-side of the MagSafe 2 connection.