Now that the presidential race is over Newsweek is reporting that the US Government, through the FBI and Secret Service, notified the Obama and McCain campaigns that their computers had been compromised and sensitive documents copied.
...the FBI and the Secret Service came to the campaign with an ominous warning: "You have a problem way bigger than what you understand," an agent told Obama's team. "You have been compromised, and a serious amount of files have been loaded off your system." The following day, Obama campaign chief David Plouffe heard from White House chief of staff Josh Bolten, to the same effect: "You have a real problem ... and you have to deal with it." The Feds told Obama's aides in late August that the McCain campaign's computer system had been similarly compromised.
This information demonstrates that the US government has a sophisticated intrusion detection capability. This is likely part of the NSA internet surveillance system that was made public by an AT&T technician in 2006.
It is likely that the system has a set of watch IP ranges that are sensitive from a national security perspective. The campaigns' computers were probably on this list. The traffic between foreign IP addresses and these watch IPs is then scrutinized for espionage. The pattern of activity flagged would be Microsoft Office documents and PDFs being retrieved or other intruder signs such as an encrypted tunnel with a foreign endpoint.
This shows that the US Government has the capability to detect some types foreign attacks although they probably have to be selective of the IP ranges they monitor. It's nice to know that if the White House computers were leaking documents to China or Russia that there is some detection capability, but the fact that this is done at the Internet backbone level means any IP could be targeted and it might not just be to look for foreign intrusions.