What rules are used during international events like the recent Samsung Cup?
There is no standardized international ruleset in Go. Generally, a tournament will use the ruleset of the hosting country.
According to an article on Sensei's Library, at least in 2004 the Samsung Cup used Korean rules.
The Chunlan Cup is (again according to SL) the only international tournament played by Chinese rules.
The Ing Cup uses Ing rules.
I guess most other tournaments use Korean or Japanese (territory scoring) rules (?).