Invisible dust on the sensor

I was wondering if after some time you use the camera some dust can create a layer on the sensor. The "visible dust" is what we see in pictures like points and is really easy to see.
Is it possible that image quality can deteriorate with the time?
I wait answers from people notice that!

I must admit I've never noticed it.

I think it may be possible for the photoreceptors or the sensor to become damaged by heat build up (calcifying crystals etc) which would degrade the image clarity, but I've never heard of it before. I've got a fairly well used 550D which is almost 18 months old (still in its infancy I suppose) but the clarity of the image from it is still as good as the day I got it. It has had its sensor cleaned once in all that time but I can't say I saw any improvement apart from the dust bunnies disappearing. Have you had the sensor cleaned on your camera to see if it improves.