It looks like MS will stop short of incorporating full AV facilities into Vista - reading about the "Windows Defender" application that will be included, it covers spyware, unwanted pop-ups, etc - I suppose all the stuff that doesn't need very intensive support (compared to AV).
Me, I will be happy as long as my favourite AV system supports Vista (AVG Anti-Virus). That way it won't cost me a penny anyway
Mind you, I can imagine some slightly misleading advertising suggesting that the built in software protects you from all known evils....