Guidance on best practices of VMware Virtual Infrastructure:
http://www.vmware.com/overview/security/best_practices.html

Security implications of the Virtual Data Center:
http://www.malanmurphy.com/Security_Implications_of_the_Virtual.pdf

Security technical implementation guide (Department Of Defense USA)
http://iase.disa.mil/stigs/stig/esx_server_stig_v1r1_final.pdf

I would also recommend to read the whitepaper VMware VI Architecture Overview:
http://www.vmware.com/pdf/vi_architecture_wp.pdf