I may have just found the answer actually. I came across another forum where someone asked the same question, and it seems the geometry is different between LGA775 and 1155. Only by a few millimetres, but that's enough to matter.
The pins on the cooler are slightly different so the stock cooler with the plastic push pins wont fit. Other than that the cooler should work just fine as you said the new i7 is a lower wattage and far cooler.
Some boards like the one i just got the asrock extreme 4 actually have 8 mounting holes around the CPU, so it can use the 1155 cooler or the 755 cooler.
I just upgraded my q9650 to an i7 37770k 2 months ago.