Out of the box, an instance will come up with IPv4. If you then perform DHCP for IPv6, you should still be able to route to the per-region archive mirrors (which only serve over IPv4 because when they were deployed IPv6 wasn't available).
If you disable IPv4 entirely, then it's true that you won't be able to access the archive mirrors. However, this isn't exactly "out of the box". :)
If you do want to disable IPv4, then at the same time you'll also need to modify your /etc/apt/sources.list to point at archive.ubuntu.com (which does serve over IPv6).
We're looking at how we can best make the "no IPv4" case work with the in-cloud mirrors, and hope to have that resolved soon.
If you disable IPv4 entirely, then it's true that you won't be able to access the archive mirrors. However, this isn't exactly "out of the box". :)
If you do want to disable IPv4, then at the same time you'll also need to modify your /etc/apt/sources.list to point at archive.ubuntu.com (which does serve over IPv6).
We're looking at how we can best make the "no IPv4" case work with the in-cloud mirrors, and hope to have that resolved soon.