I'm part of the IP Admin group of a large international ISP and have seen this firsthand. New customers routinely ask us to route space, and sometimes it's difficult to tell if it's theirs or not what with all the mergers, acquisitions and renaming of companies. There's definitely more scrutiny of these requests than there was a year ago.

A few months ago spammers started to hijack IP space that was registered to companies that are now out of business, which means that most likely nobody is going to notice what they've done.

After a while it's almost like getting squatters' rights - I've been using it and nobody else has a real claim to it, so it's mine.