Very good question. I've been thinking about a central way to manage keys. There doesn't really seem to be one, and it would be a big point of attack because an attacker might abuse it to add their own key.

Right now what I do is I just log into my servers and copy the list :P I don't have that many anyway.

The openssh people are also advocating certificates now, which means you'll have to set up a PKI, which will take care of revocation and such.