Keyword Density: How Much Is Too Much
This thread, started back in August but recently revived, at WebProWorld starts off pretty well but 2/3's into the first post takes a dramatic nose dive as site admin CRich starts talking about sites being banned for too high KW density. If you can filter out the wild conjectures and outright rubbish however, it's a good read. Has some nice links to other threads at other forums also.
Spam can occur when your density exceeds what is the necessary amount to gain a respectable ranking.
Heaven help us all.....