I've had the same problem with some images from my Powershot S30. All images were taken at full resolution and no other setting were changed and I have not been able to figure out why the resolution was chnaged (randomly). I normally use an action to set lvl's, curves, saturation, etc. so I simply inserted a command in the beginning that set the resolution at 300 (or whatever you want). I would be interested in finding our why this happens but my work-around solution functions well and I don't even notice it.