Here's a funny story: How The Google Police Nabbed Me For Paid Links
That's something I used to fear every time I talked to a potential client. I was wondering if I'm talking to someone who is merely pretending to be interested, but was actually working for Google. :p
Of course, I did get penalized back in 2007 from PR6 to PR3, and it barely changed anything. Sites even bounced back some time later to PR5 where they're kinda stuck now.