The U.S. military's role in South Korea as of 2011 is primarily logistical support and air cover.
If an invasion happens in 2011, the South Koreans will be doing the majority of the fighting, with the USFK in the background supporting them.
However, back in 1991, the USFK was much more battle capable in handling a North Korean invasion, primarily because the South Korean military was not as developed as is today.
It's one of the reasons why so many in the U.S. support withdrawal from South Korea, as the South Koreans are more than capable of repelling any North Korean invasion without USFK support.
I don't know if any of you on here know this, but in 1994, a Second Korean War was very close to reality as the United States was mere hours from launching a strike on a North Korean weapons facility, possibly restarting a war. However, the strike was aborted at the last moment, due to a meeting between former U.S. President Jimmy Carter and North Korean authorities.