At first, I just thought it was an issue with my browser or bandwidth - perhaps I needed a quick cache clear or reboot, but I realized quickly that ALL Advanced Access websites were totally offline today. After doing a quick search, I found news at Active Rain that indeed, AA were having some "power" issues and needed to relocate all of their servers to another location.
Of all the various scenarios I've heard over the years with multiple hosts, both big and small, this is a first for me. I've heard the "denial of service" attack, we're working on it. I've heard the "close your browser and reboot," it's all your own fault excuse. I've heard the "you're using unsupported xyz." IMHO, it just seems a little fishy.
There's an unsubstantiated rumor over at Active Rain that they're moving servers to obtain new IP's due to spam black listing. If all of their IP addresses are black listed from most servers due to spam, few of their subscribers would be able to get any email through to anywhere.
Follow up - May 24th: It's official - on behalf of a client who has an AA website and Comcast as her ISP, we contacted a rep at Comcast because she has had serious email trouble. It seems Comcast blacklisted the entire Advanced Access range of IP's due to spam issues. The rep at Comcast stated that the blacklisting of all AA sites wouldn't be listed unless the management at AA contacted them directly to work out the issues.
So, you can firmly bet that the "power outtage" is a cover story for AA moving their entire server base to new IP's - however, it may be a short-term solution if Comcast quickly detects this move and simply blocks the new set of IP's. Be warned!