It is explained in the opening paragraphs here[1] but I'll explain.
There are two ways to make AMIs: EBS-backed or instance-store. For instance-store, you basically need to debootstrap a chroot or something like that. You can do this offline, technically. EBS-backed you just launch a source AMI, build on that, then re-snapshot it.
For the purposes of a 0.1, Packer currently only supports the EBS-backed approach. This is not a technical limitation, however, and in a future version I do want to add the ability to make instance store.
No, that's not planned. I've actually not seen/read anything that does this. If it is possible, it is certainly interesting, but I have a feeling that this would be very tricky.
There are two ways to make AMIs: EBS-backed or instance-store. For instance-store, you basically need to debootstrap a chroot or something like that. You can do this offline, technically. EBS-backed you just launch a source AMI, build on that, then re-snapshot it.
For the purposes of a 0.1, Packer currently only supports the EBS-backed approach. This is not a technical limitation, however, and in a future version I do want to add the ability to make instance store.
[1]: http://www.packer.io/docs/builders/amazon-ebs.html