barone:On demand = Is the term given to a service that is available as and when you want it all you need to do is turn it on. Like the new on demand tv services popping up now, watch a film as and when you want to. When applied to IT security this term basically means 'off your network'. An on demand service is delivered at the internet layer, therefore, taking away ANY need for hardware or software on the infrastucture.
It is very popular in the private sector because it removes the complexity of having to deal with the problem(re:spam, av, email content, web access and content, web threats) and is backed up by SLA's(service level agreements).
SurfControl's on demand email service, Mailcontrol, carries a number of SLA's 99.999% availability, 95% spam removal, 100% virus protection etc etc.
Hope this helps....