I manually invoke Ping-o-Matic after I write a new post on TrenchMice’s blog. It’s supposed to happen automatically, but doesn’t — something’s amiss with our WordPress installation, and it hasn’t been important enough to debug.
Great service, every blogger uses it, and it’s worked fine for months. But I started to get either session timeouts, or the “Slow down, you’re pinging too often” page, starting about five days ago.
I informed the Ping-0-Matic powers-that-be. I did it via a comment to the most recent (but still quite old) entry on their blog, because I couldn’t find a contact address. I checked back a day later and my comment was deleted, but they never replied back. Maybe they already know about it, and I’m the Nth person to ask.
Seek Nuance 
Two months later, and I’mm having the same problem — and took the same tack you did: left a comment on that ancient blog post you cite:
“It’s April 14, 2008, and I’m getting ‘Slow down cowboy!’ every time I ping, for three different blogs, none of which ping automatically (I double-checked to make sure my settings hadn’t changed), and all of which I had NOT pinged in (respectively) approximately 11 hours, 18 hours, and three days. Methinks the bug is back.”
We’ll see if my comment is approved.