The Cisco UC Product Suite consists of a slew of different appliance virtual machines, providing functions like traditional call control (dial-tone), voicemail, presence, E911 (specifically, providing more specific location information to the Public Safety Answering Point than just physical address), conferencing (think web-based content sharing and audio bridging), and telepresence (video conferencing). All of those functions run separately, so there are no co-resident applications on a single virtual server instance. On the physical host multiple virtual machines are allowed, however over-subscription of resources is not permitted. For instance, if each virtual machine takes 2 processor cores and 4 GB of RAM, a physical host that has 2 quad core processors (8 total cores) and 32 GB of RAM would have a maximum of 4 virtual machines. Even though there was more RAM available, the physical host ran out of available cores, and thus would not be able to hold additional virtual machines.
As far as scalability goes, Cisco is very rigid in their product system requirements. This means you can't just go willy nilly and throw a product instance on just any old server or hypervisor. The general rule of thumb is that their products should be on UCS servers, virtualized on VMWare 5.x, and if there is any SAN connected to the servers it should be connected via Fibre Channel. However they will allow customers to go outside of these requirements to some degree. You may use any brand server you wanted as long as the hardware specs meet the requirements; and you also may use iSCSI or NFS for storage. The caveat is that the level of support you would get from Cisco would not be as good as it would be if you had stayed on UCS servers and fibre channel or direct attached storage.
At first, customers didn't have the option to virtualize, so every appliance would run on bare metal machines that were OEM'ed from HP or IBM by Cisco. Version 8.0 introduced the option for doing the hardware abstraction, and now virtualization is going to be the only way these appliances are supported in the upcoming 10.0 release. There have been rumors that Cisco would open up the hypervisor requirement to use something else. Personally, I'm hoping for some KVM support, because today VMWare is just another thing that customers have to purchase.
Thanks for reading, and stay tuned for part 2!