The Penske run RS Spyder program was often criticized for really being a "works" effort. (Especially when it was beating the R10 on the tighter ALMS tracks). The ACO hinted that it was not in the "spirit" of the regulations and then proceeded to apply restrictions on the LMP2 class (read RS Spyder) continuously until it was really struggling to be competitive at the very end.
Of course, what the ACO often forgets is that there is no such thing as the "spirit"of the regulations. They made a set of rules which Porsche exploited to the max (as all good manufacturers do) and built a car that was able to challenge for overall victories.
Later the ACO actually put in the rules that the LMP2 class (along with the GT2 class) are intended for private teams and not manufacturers. the "excuse" is that the manufacturers drive up the cost.
Whilst this is certainly true (and difficult to argue against) I am not sure that real intention behind it was to make sure that the diesles could regularly demonstrate their dominance over petrol. The ACO knew that a manufacturer like Porsche would only race a petrol engined car, and so restricting them to LMP1 would have guaranteed that the diesels would have demolished their entry at every race.
The non-diesel manufacturers, understood this of course, and that is why you have not seen any other manufacturer (except AM) entries in a class reserved for manufacturers during all this time.
The ACO has finally realized the game was up (but they managed to put it off long enough for a diesel Peugeot to win Le Mans), and so in 2011 the rules should be much more balanced.
Until then, expect to see diesel dominance.