OK, this is clearly a bad thing, but I don't think it means that your private LAN is immediately accessible to people all over the world does it? Multiple routers using the same keys means you could be tricked into logging in to someone else's router without knowing, but that would still require some way of directing your traffic to the impostor's device to begin with, such as DNS hijacking.
Finally, a breath of sanity... Thank you, nuckfuts! A shame this is the bottom thread in the post.. at least when I got here.
There is a huge difference between a host key and a user key. These consumer devices all share the same host key, which is only used by the client to verify that the host you're connecting to is the host you think you're connecting to. This is the key in /etc/ssh/ssh_host_rsa_key for those with access to a Linux shell, and is never encrypted or password protected. How do I know this? Because there's no way to determine what user keys are in a host's authorized_keys file with just an unauthenticated connection. However, when a client connects, the server always sends the host's public key along with a challenge signed by the host's private key.
The host key is only ever used for authentication, never for authorization, which is to say it identifies the server you're connecting to, but in no way grants any privilege to access it. The only risk here that I can think of is a MITM attack. Since the host key is well known, someone could fiddle with your DNS or local ARP tables and make a victim connect to their evil server without the scary "MAY HAVE BEEN COMPROMISED!!!" warning you get when the destination host key doesn't match what's in the known_hosts file.
If someone can paint a more frightening scenario (based on known host keys, not user keys), I'd like to hear it. If you don't understand the difference, don't bother trying.