Thank you everyone for the inputs!
I actually think this rule is pretty important comparing to some rules of composition which IMO are quite subjective. This rule prevents the undesirable blurriness of pictures.
Bjorn van Sinttruije wrote:
Make sure you'll multiply your focal length with the cropfactor of your camera though
I certainly forgot that. Thanks for the heads up!!!
One last question, do you know if ISO speed will affect this rule?
Take Olympus ZD 50mm (100mm film equiv.) for example, I need at least 1/100 second to get a clear handheld shot, but does that apply for ISO100? If I increase the ISO speed to 400, how much time would it be now??