I see people talking about this all the time, but I have never seen any real data on this "issue". Obviously having 1000 spammy sites on one IP will raise some flags. Actually if you are on shared hosting, like Hostgator you most likely have over 500 other sites on the same IP/Server.
I belive the IP issue is smaller than people tend to say it is. In a SEO perspective anyway. I've been top ranked MANY times in medium competition niches, on shared hosting with thosand sites on the same IP.
I've also been doing autoblogging for a while, and I've had over 50 blogs on the same IP. It has never been a problem, and I'm sure i could go to over 100 and running just fine.
This is my personal opinion and thoughts, and I'm not a SEO expert at all.