Thank you very much for reporting this!
As your google search pointed out, this spammer has visited all of the largest guestbook systems. (The system has not been hacked in any way, just spammed. The system was not hacked last year either, but a severe bug was discovered that let a single user accidentially alter guestbooks.)
Almost 400 guestbooks at UltraGuest.com were spammed over the past 24 hours. Even though all gustbooks have spam protection features enabled, this spammer was more difficult to dected for a number of reasons including use of different IP addresses for each entry.
I have removed these spam-messages from the system.