B737-300 flight crew missed a crossing restriction while descending in level change mode and failing to adequately monitor flight progress.

Narrative

On arrival; we were issued and acknowledged crossing at the intersection 250/11;000 and then continue the descent to 6;000 FT. The Captain was the pilot flying and selected 6;000 on the MCP and descended in level change. Approaching the fix we got a runway change to the left runway (we were expecting the right and eventually the runway was changed back to the right for landing). Both pilots were heads-down approaching the fix programming and verifying the left runway. As the pilot not flying; I did not realize the Captain was descending in level change (we had been in VNAV initially; but switched to level change at some point in the descent to the intersection). About four miles out from the intersection; I noted that we were descending on schedule to cross the intersection at 250/11;000. Just after we crossed the intersection; ATC called and asked if we had instructions to cross the intersection at 11;000 FT. I noted our altitude at about 10;200 FT and we were about a mile inside the intersection on the arrival. After landing at the airport; Ground provided a number for the Captain to call ATC for a possible pilot deviation. Descending in VNAV would have prevented this possible deviation. As the pilot not flying; I should have been aware of the pitch mode used in the descent.