What follows is not a real answer (I believe the accepted answer is correct), but more an interesting side note.
Sai played go on internet. But Sai is a purely fictional manga character.
However, in 2003 - 2004 on a real go server (KGS), a strong opponent appeared and had a 100% win ratio. He won also against some professional players. His name was "Tartrate".
Actually, after some time he lost a few games, but these losses were on time, or occurred during simultaneous games. Note that KGS is not really a "strong" server, so that even if the best players on KGS are professional, they are not the best professional players. But still, Tartrate managed to get an impressive winning streak.
The identity of Tartrate was kept secret, and actually it was only revealed in 2009. So at that time (2003 - 2004 and later until 2009) many players wondered who he could really be. There were many gossips and a web page was created in order to figure out what could Tartrate's real identity be.
Some people even claimed he was Sai (more as a joke than a real hypothesis).
Nobody would believe (seriously) this user was a computer bot. The reason is that at that time the best bots were still at kyu level.
Even today (June 2013), the best bots are at 4 dan or 5 dan amateur level (which is still weaker than an average insei for example).
Also, another interesting anecdote on this topic is "the shodan bet". An amateur player ranked first kyu made a bet of 1000$ with a friend that he would not be beaten by a computer player before 2011. He had to play a series of games against a computer in 2010 and won the bet. However, in 2012 (after the bet deadline), he lost 3-1 against another computer.
The web page about this bet gives some information about computer bots.