I recently discovered I was conned into linking to two websites in a bad neighbourhood (a scam and a phisher) by a would be advertiser after I started selling exposure on my website last year. At the time I'd had authority status for years and I used to dominate certain SERPS both of which were badly hit (half PR and lost A.S) although I initially thought this was the result of some rather crass mistakes ("sponsored links:", no rel="nofollow" & no javascript). Anyway I can't help wonder whether my SERPS weren't usurped by a rival and whether this sort of thing happen a lot...