Great point. and Intel's and AMD's black box RNG generators (RDRAND etc) provide entropy... but they're thought to be compromised (which would be nearly impossible to detect). At least other firmware attacks would/should/might be detectable. By definition, RNG's produce unpredictable output -- detecting predictability is much harder than with other potential attacks (versus, for example, the sum of two numbers, which should always give the same result).

So, FreeBSD and Linux both use hardware RNG's as entropy inputs and mix-in other sources of entropy as well (which hopefully mitigates any loss of available entropy and also adds in other believed-good sources of entropy such as timing, network traffic, etc).

Ubuntu is using a new network-attached source of entropy which is itself constantly reseeded with the network traffic used to access it. (There's some inception joke there somewhere..) Of course, you may not be able to rely on the SSL/TLS connection that you're using to access it, so you might be seeding with an attacker's steady stream of 1's...

Of course, getting access to real hardware entropy in a hypervisored or virtualized cloud server/instance is the second part of the problem.

The third part is to make sure that SSH server and client keys are distributed properly. That's what Userify is for, but it really only helps with the client/user keys.. it doesn't help with server keys. (Yet?)