Cisco, Network Associates, Symantec and Trend Micro have launched an initiative called Network Admission Control, which, cleverly enough, won't let any client into the network which isn't stacked with the latest antivirus revs, patches, etc.
In case it isn't completely transparent, they stand to gain a lot from this initiative - not just in terms of short-term profits, but also in term of a massively locked-in user base, and automatic consumer share boosts.