The dry nose = illness thing is an old wives tale. It just means your cats mucous membranes are dry. This could be for many reasons. Its probably that you've been running the heater in your house and drying out the air, hence, a dry nose. It might also indicate dehydration, but there would be other signs like skin tenting, lethargy, scruffy appearance, etc.
I agree with Kobster. A dry nose, in and of itself, means nothing. If a cat's nose is dry, you need only worry if you see other signs of illness (lethargy, loss of appetite, etc.). If it's a simple case of an occasional dry nose, then you have nothing to worry about.
With dry winter air, I find both my cats have dry noses. As others have said dry noses mean nothing. I would be worried about a nose that has any kind of discharge. A clear discharge usually means a slight respiratory problem or allergies. If it's not clear, it's an infection and requires a vet visit.