You can do insane dunks at almost anytime in NBA 2k8 also.
mabey on easy, i play on the hardest diffculy setting with the sliders tuned to help the defense.
IF you try to just get to the rim with no play on, you'll get stripped or get called for charging 90% of the time, because unlike NBA jam errr... live, players don't turn on a dime from a full spirnt, they actually have to plant and pivot, and the computer is good at taking charges or fouling to prevent the dunk, as well as bodying up if you take to long to make a move.
Online, if you know what your doing, its easy to keep an opponent away from the rim unless he runs a good play or has a fast break.