This could be in the 'Ask a scientist' thread It's caused by the wingtip vortex - the wiki is quite good for this particular subject - nice pictures too!

Basically... a vortex is created at the wingtip of any aeroplane. The white trails are the water vapour in the air which condenses as a result, forming long 'clouds'... Different weather conditions (temperature, humidity) influence the creation of these trails, which is why some days you'll see none, on other days the whole sky is full of 'em.

Edit: And it could be contrails!