Mine did the same thing, the image became very dark after about 200 hours. I was not happy, but sent it back and Sony replaced the bulb with no charge. I noticed they also set the high altitude mode which causes the fan to run more often (even though I'm not in high altitude location) and it's been fine since then, I think I have about 800 hours on the replaced bulb. I was extremely frustrated at the time, but Sony did take care of it and the turnaround was very fast. Definitely make sure high altitude is set so that the fan runs more often.
Also make sure you aren't cutting the power to the projector when you power down the system -- the fan needs to run several minutes after the projector light turns off, if the fan doesn't run, the bulb will burn out quickly. I put a UPS on the projector only to prevent this issue since we have frequent power outages in our area (not sure why)