Heres a theory.

Itachi's special ANBU mission involves tracking down Oro and bringing him to justice. While Itachi is away, Oro comes in and slaughters the clan. Itachi returns just after and see's Sasuke coming home from practice (the pole scene)

Itachi does not want to show Sasuke that he is weak (Itachi failed to protect the clan and let everyone die) so thus creates the illusion that he did it himself.

This theory is flexable since it doesnt have to be Oro to have killed the clan, it can be anyone. I would also like to introduce a sort of twist in to the plot in which Itachi and Sasuke's father actually betrayed the clan. (Their father doesnt really seem to be a nice guy from what we've seen)

His purpose was to get Itachi away from the village (as he is so strong) and thus the killings can happen. When Itachi returns he see's his father killing his mother and quickly binds him in some sort of manner. Itachi then kills his father execution style for betraying the clan and sasuke walks in just at that moment..

Okay I got to much time on my hands to think of all this but I think its a good theory