We're looking into having an extranet for SharePoint 2010 on a hosted server.
Aside from sharing the resources of the physical server, are there any concerns, especially about security, in using a virtual server over a dedicated server?
The only concerns you will have are the shared-performance issue. Assuming you are referring to a VPS or a virtualized server, each virtual server is completely partitioned off from the "host" server which is managing each of the virtual machines. Security on a virtualized server would not be any different than it would be from running a dedicated server.