@sky
i thought about this also. my first line appeared after i converted a final cut movie overnight to mpeg4, modified the export setting and exported again the next night and so on for about a week. this wasn't actually the first movie it ever had to convert.
when the powerbook is about to process movies etc. for several minutes or hours (overnight), i switch on the ventilators of the stand where the powerbook is put on. actually i never did that to save the display from heat (i'd never thought that would be a problem), i did it because i had two dead harddisks in the last two powerbooks and wanted to avoid a 3rd dead one.
the other lines just appeared more or less during normal day work, some webbrowsing, email, documentation, ...
i hardly play any 3d games on the powerbook, and not even the converting of a movie is related to the graphics processor, so i don't see any connection to the graphics chip (talking about my powerbook only, of course).
i think there are many possible "reasons" one can think of why the display breaks. in the end this results in "you used it, that's why it broke." and sorry, i cannot accept that 😉
fact is, there are MANY displays breaking within the same laptop series. i've not heared about so many defective displays in the g3 powerbooks, macbooks, g4 12"/15" powerbooks. to me it's obvious, our 17" displays are of bad quality (alternatively there's a construction problem with the display cable layout, like somebody mentioned here) -- and this is not a reason why we as customers have to pay for that. a top-end (and expensive) laptop should work for more than 2 years.
and, looking at how much money apple made in the last quarters (years) i really don't understand why they don't recall these machines for repair. this would improve their good image. instead, apple is becoming an unappealing company, don't care about a few long year customers if you can get a 100 new ones buying an ipod or mobile phone. maybe there's a reason why they dumped the word "computer" from their name.