It was previously believed that komi was first introduced around the middle of the 19th century, when players like Honinbo Shuwa and Honinbo Shusaku tried it in relay (team) games. They apparently could not decide on whether 3 points or 5 points was more appropriate - Shusaku favoured 5.
But a book discovered not long ago in Gifu shows two games from the Horeki era (1751~64) between two very strong amateurs where komi is given, But the term used is kaeshi (giving back) and they gave back 3 points.
In this context, for 'amateur' we should read 'gambler' rather than 'weak player'. This association with gambling may be why the official go families avoided it for so long.
One famous gambler was Shinomiya Yonezo (1769~1835). Backed by the Lord of Awa, he is said to have earned 3000 ryo from go gambling. Converting that to modern prices on the basis of the cost of rice, he was a dollar millionaire.
More details of the history of komi are on the GoGoD CD. The rise of gambling go is also discussed in a long feature on Genan Inseki to appear in the Summer 2005 edition.