Same problem here. I'm at the stage of trying to understand the problem, and have read all the above posts. The key has worked fine in the past.
When I saw the link that burnetts posted, I couldn't get it to work in my browser, which is through my ISP, and is different from the webhost. Unfortunately, I cannot SSH into the webhost account; only access it from CPanel, and trace is only allowed there for email addresses.
Running a trace from my computer:
Trace rest.akismet.com failed, no such host
(note that http: and // have both been snipped so the link isn't live)
I'm thinking that there is some sort of larger DNS issue this morning.
Should anyone be able to do a trace from their PC to rest.akismet.com and should a lookup occur?