Clustering Sites and IP Configuration

[Little Geek]

I know this is a huge issue in SEO, and I would like to get some expert advice and information that we can compile for our network administrators as to why it's important to keep IP's separate. We have 80+ subsidiary sites that we manage and the network administrators are in the middle of clustering the sites together to optimize our space, and so it's easier for them to manage the sites. However, in the midst of all that, they want to put all the sites under ONE IP address.

Tell me your thoughts, what you have seen that is detrimental or positive, the pros and cons for each of the search engines that you have come across, and what you feel might be good solutions to this problem.

Thanks!