I don't think we'll see any change to AV on the desktop. That's not a fast-growing market. Enterprise-grade security "in the network" on the other hand is a multibillion dollar market, and growing fast. Signature-based detection is still the premier way of blocking threats like email spam and malware. In order to scale to the higher bandwidths that today's networks run at, it's necessary to use purpose-built hardware for network and regex processing.