Apparently ebay is the biggest market for stolen bike bits (and bikes). Many end up in eastern Europe but a hell of a lot are being nicked by neds to thrash around the local bing / spoil heap / country park.
When I used to stop people on motocross bikes in the country park I worked at most of them had the frame and engine numbers ground off. The police could only sieze the bike if they caught them red handed riding the bike. Most bikes they did sieze would end up getting crushed as they could never trace the owners.
This is one of the reasons I'm reluctant to get a modern trail bike as it just makes you a target....