I recently read another cat lover's blog on the possible risk to domestic cats.
I was intrigued so went to the WHO website - no information there apart from this old report. Then I found a good blog here.
A bit of hunting around turned up the two mentioned studies - here and here. The risk DOES EXIST and can easily be fatal to cats. Here in southern Spain there is a large feral cat population which would make the spread of the H5N1 virus both fast and easy in the domestic cat population.
I think I'll be keeping the cats in from now on too.