The legal concept of plaigarism is a seriously tricky legal area.
There is actually no legal defence in the claim that a copy was entirely subconcious - as George Harrison tried - and failed - in his court case -
http://www.benedict.com/Audio/Harrison/Harriso n.aspx
so if the court is presented with music that is in the public domain - and obviously it will be - then Coldplay are looking at loosing their case.
There are actually lawyers who specialise in this type of action - because the rewards, and costs, can be considerable - no doubt both sides will wheel in their own experts to jaw over it all.
As I said, saying you didn't mean it is no defence in law, so we will see, but Joe might be trousering some serious wedge from the Coldplay coffers if things go his way.
Oh, and yes 4getmenot - Vanillla Ice did sample wholesale from Queen and David Bowie, and got legally spanked for his trouble - a career-stalling manouvre as it turned out, so not always a bad thing to get these issues before m'learned fridends.