there was a recent thread [webmasterworld.com] with similar observations. I also reported (in yet another thread) that my backlink profile contains thousands of links that were created via hacks... I have no control over those, and I believe they have had a negative effect on my rankings (unnatural link warnings + Penguin). My disavow list now contains 8,000+ links of this nature. From time to time, I see those hacked pages showing up on competitive queries. They redirect to spam/lead generation forms, and as Tedster pointed out, they don't seem to stay in the serps for long; however, new ones are always appearing.