I guess it depends on what the "core" of the MS AntiSpyware functionality means. IT could mean that just some basic spyware behavior prevention/detection stuff will be included as well as an API to let AntiSpyware software integrate into the security center.
They could then charge for signature updates with OneCare (or you could use third party software).
The fact that there is an AntiSpyware item in the Vista security center doesn't mean that it is only for MS's AntiSpyware. Remember that the Antivirus monitoring item just above it already exists in XP sp2 and works with 3rd party AV software.