I'm positive that U2 leak songs on purpose. Remember when Bono gave Jo Wiley a copy of "Electrical Storm," knowing damn well that it would make it to the airwaves and generate some excitement?
They're savvy marketers and they know that "all press is good press." That story about the "stolen" CD in France was just a way to drum up hype for the album. There's no way that CD really got stolen. Same thing with last week's release of the album to the Internet, along with "Mercy." U2 is smart enough to know that when CNN runs info on their on-screen ticker about the new album's "leak" to the internet, it generates more buzz. Buzz eventually translates into more press, more hype, and more sales.
I think it's great that they do it, mind you!