Hi,
Virtual is not the only form factor that Brightmail Gateway runs on, we do also have the physical hardware appliances running BGW. Obviously, there is a cost associated there also.
ESX is generally an attractive medium (in both cost and administration overheads) for customers that are already at a production stage with virtualistion in other areas. Purchasing blades/hardware and licensing ESX directly for use with BGW might not work out for customers using their first hypervisor environment.
We recognise that and on the back of customer requests are in the process of certifying ESXi.
For test purposes, the virtual machine can run in VMWare Workstation or VM Fusion if you wish (although be aware of performance) or if you want to check it out semi-server grade the free VMWare Server can be used (but not supported for production use, in line with VMWares own support policy).
You can probably even run it through VMWare Player if you just want to get a hands on view of it.
Firewall based AV and AS are *generally* either open source or OEM'd from vendors (for example Symantec OEM software out to many, many vendors), obviously without knowing what you are referring to I can't really substantiate any incentive/differences. Eitherway, OS or OEM has a lead time associated with it, not only for updates but also in some instances rulesets, which are commonly not as effective as Brightmail anyway.
Anyhow, this isn't a competative or sales forum so I don't want to push the "we're better than X" subject in this thread, that said - i don't want to cut the conversation short so feel free to give me a shout on ian_mcshane@symantec.com if you want to go over your options with BGW.
Cheers,
--ian