According to a team of Japanese scientists, tripods under 3.3lbs can actually worsen camera shake.
Nishi Lab and Tani Electronics co-developed a new tool/method to measure the effects of camera shake. What they found is that SLR cameras lose 1/4 of their resolution to mirror shock—the vibration from the camera's internal mirror bouncing up.
To make matters worse, the team discovered that this mirror vibration can be exacerbated by mounting the camera to a tripod under 3.3lbs.
Of course, even a light tripod can thwart general human shakiness. So while a tripod may enhance mirror shake, it's still probably helpful in eliminating human shake. And I'm guessing the human shake is a bigger problem for most amateur photographers. [Tech On! via Crave]