I took the time to write it down this time. The popup says:
The page at "http://fileserving5323.s3-website-us-east-1.amazonaws.com" says:
Check your ANDROID NOW for a VIRUS!
I also noticed that it came up the same time as an ad banner for, "Free Android Antivirus & Security" so I am guessing it is attached to the ad banner.
I don't mind the relatively unobtrusive ad banners but this seems a bit much, especially for a registered forum supporter to have to put up with.
If any of the Moderators / Super Moderators would care to address if this is in fact an ad banner device, I would appreciate it.