diberry - 2:50 pm on May 9, 2013 (gmt 0)
Thanks, tictoc. I talked to someone in a position to know, and they said it's not clear whether software like that is against the TOS or not.
I will wait and see, though, because none of the WP plugins I found seemed quite right - they're either insecure or have had trouble with their API's getting shut down. I'm now more likely to go with a pinning service like Pingraphy, but will still wait and see if their terms change.