This has come up quite often during the time I've been on the C25K forum and usually from folks that are struggling to breathe in and out through their nose as per the podcasts.
This article in the Guardian Running blog focuses on the whole breathing debate and comes out firmly in favour of mouth breathing as the way to do it!
theguardian.com/lifeandstyl...
If you haven't come across the Running blog in the Guardian before there are plenty of other articles worth reading too