Welcome to WebmasterWorld Guest from 184.108.40.206
An interesting test would be: From that machine, using the ISP that doesn't see ads, ping pagead2.googlesyndication.com
If you don't get a valid response, your ISP or your ISP's DNS (or whatever you have your DNS set to) is doing something funny (but I doubt that would happen).
I didn't say that the ISP was blocking it. I said that ads were not appearing when I was dialed in to that ISP. That's entirely different.
Anyhow, they are back today.
My guess is a Google glitch. Maybe they thought the ISP was a proxy. Who knows.