First off, I would like to say that I am not a Coldplay fanboy who is just getting annoyed about how people complain about my favourite band. I think Coldplay are an average and overrated band, who have made some good songs on each album, but an equal number of woeful songs. Radiohead however are possibly my favourite band of all times, and in terms of innovation in sound very few compare.
It is a very well published idea that Coldplay 'copied' Radiohead. Watch any youtube video of either band, and somewhere down the comments this debate will appear. The same also goes for Muse, Travis and any band of this new, slow, falsetto based alternative, (something I think should have a genre of its own but that's a different matter.)
You see, what's key is the difference between copying and influencing or inspiration. Radiohead are a major, or perhaps, the major influence of Coldplay and Chris Martin happily admits that. All of their work is heavily indebted to The Bends and OK Computer, 'High and Dry' is the song often used for this. But this is just influencing. Coldplay heard this music driven by acoustic guitars and falsetto, loved it and so made his music based on it. Their voices are undeniably similar (obvious Thom Yorke's is better) but I can tell you, Chris Martin would sound very different had he not picked up Radiohead. This is just influencing a style, and the exact same happened to Thom from Jeff Buckley.
Copying on the other hand is directly taking someone else's work and using it in your own. So the countless bands who stole from Blues musicians were copying from them, not being inspired. But as Coldplay have a similar style yet nothing else, they were influenced by Radiohead. There are never the same chord progressions or bass lines or anything of that sort. It was simple plain inspiration.
And I know there will be the inevitable people in the comments saying 'this is pointless, everybody knows this', but the debate is still raging and the sooner it stops the better as it's choking good music.