First, he claims that Coldplay's song, 'Viva la Vida', uses the same chord progression as his song, 'If I Could Fly' . So what?! There is only a finite number of possible chord progressions that are actually listenable to. How many "hit songs" have used a 1-4-5 chord progression? This progression is the basis for nearly every song Green Day has recorded, and is at the foundation of most popular music. Musicians are bound to build of off other musicians' songs, either developing slight melody variations, or by writing their own lyrics. If Coldplay did borrow from Satriani's song (they might never have even heard it), 'Viva la Vida' is still unique, at least by definition of the word.
Second, what does Satriani hope to gain from this lawsuit? The acknowledgement that Coldplay ripped him off? This would seem to be more of a compliment than anything else. Does he expect to receive royalties from the song? How exactly would that work out? Satriani did not pen the lyrics to the song, nor is he responsible for the extensive play the song has received. Most of the song's popularity is derived from the band's past success; it is popular simply because it is Coldplay's latest single.
Joe Satriani may have a valid claim against Coldplay, however, this lawsuit seems to be overkill to some extent, and may end up being little more than a publicity ploy for the comparatively unknown guitarist.