The judge has erred in his judgement in making this ruling, the data thats accessible across a p2p network is not files in actuallity its meta-data and that may or may not mean that there is a file relating to the received meta-data, but I do believe that such a meta data result should in itself give grounds for a warrant as in real life its unlikely that folks would share meta-data for child porn unless they either have it or are acting as a honeypot for those that seek it.
There is of course a further issue that crops up here and one that can have a serious impact on innocent folks, its possible as we all know on this and most other p2p networks to spoof such met-data results, knowing this makes the judgement seem even less well thought out.
I wish judges would do their homework or ask for professional assistance before making such judgements.