Confusion regarding the intent of another aircraft in the traffic pattern resulted in a traffic conflict for a Cessna 172 pilot.

Narrative

I was approaching from the west and had contacted the tower and was advised to enter a right downwind for Runway 24L. I was further advised that I was number 2 to land. Looking for the other traffic; I saw what appeared to be a Cessna 150 or 152 that seemed to have just taken off from one of the Runway 6's. When I saw the other plane he was heading away from the airport on the runway heading and was already at a higher altitude than I was. I contacted the tower and advised that the other aircraft appeared to be heading away from the airport and received no response from the tower. Since the other aircraft was now some distance away and was still climbing; I assumed he had changed his intentions. I turned to base; keeping an eye on the other aircraft who continued to climb away from the airport. As I turned final; the other aircraft dived for Runway 24R; almost in an acrobatic fashion; and passed about 3500-4000 FT in front of me and then landed. It was not deemed close enough for me to take evasive action but I advised the Tower that the other plane had just passed in front of me. The Tower responded that I had cut him off. I subsequently landed uneventfully on my assigned runway. It seems to me that two actions could prevent a recurrence. First; on my part; in a similar situation; I have resolved to continue my downwind until confirmation from the tower that the other pilot has abandoned his landing or until he passes abeam of me. On the part of the tower; it would seem that insistence of adherence to established traffic patterns would remove any ambiguity from the minds of those attempting to land or depart.