The next version of Macintosh operating system, 10.4 code named Tiger, defines a default RSS reader at the system level. Preferences in the current early start kit allow a user to define the time between feed updates, the color annotation of a new article, and when to remove stored items. Hopefully this means aggregators will be able to share a common feed storage location.
Update: MacNet took down at the request of Apple Legal. The preference pane showed a drop-down boxes to select the default RSS reader, choose an update interval, highlight new items, and the length of time to store items.