They've been around for a few months, but like all new AWS things, they're being slowly rolled out across regions, so may not be available everywhere. Flick through the 'region' dropdown in that pricing link to see if they're available where you want them.

The micro went from 0.6G to 1G ram in the 't2' generation, and the nano fills the need people have for the old micro size. I use them for bastion/nat boxes :)