This is an annoying feature with CA fill, and the usual advise is to try again, but there is a workaround. Select _only_ those areas that you want to be considered for the CA fill sampling, and copy them to a new layer (ctrl j) Now use CA fill on that new layer, and it can only sample from the pixels on that layer.
Many thanks, Trevor. I tried your suggestion, which is useful to know. The result is better than above but if it was for a job of work I'd have to cut, paste and clone. Perhaps there isn't enough distinction between the area to be replaced and potential background for CA to work (normally I've had situations with definate edges and shapes). I'm using CS5; do you know if it is addressed in CS5.5?
Happy New Year!