It sounds like an interesting idea with some real problems to solve. I'm not a professional sysadmin so wonder what the current solution is? That would be a great source of information which many people on here could provide.

My #1 concern is trust. Would a webdesign agency really trust all their SSL certs to Silo for $50/yr? Centralising that number of SSL certs at an random online service seems a little insecure. I'd probably prefer a truecrypt-encrypted USB key, or an encrypted volume of dropbox/whatever or something like that.

I wonder if there's a business here in just the SSL expiry notifications? It seems a common problem which is theoretically solved by Google Calendar but obviously no one actually thinks of a general tool for the specific purposes of reminding them about expiry dates.