I still think my D50 gives excellent results - however it has recently added a new feature which is only detectable on certain images (usually dark ones) of when showing pictures on a big screen or the computer.
Basically I have thin line down the middle of the image - starting about a third of the way down - with the top of the line showing as a round dot.
I have heard of the sensors etc being cleaned - but this sound more serious.
Elsewhere people have suggested getting an authorised dealer to send to nikon for assesement - but as ever - difficult to asses how much it is actually worth spending to fix.... given that the camera is 5 years old and I have seen them on sale for £150 on ebay. I guess spending say £100 might be worth it but any more than that I might as well just save the money towards a newer Nikon (body only) like the D90 and D7000 amongst others.