Dave Winer asked whether Longhorn will have an RSS aggregator built-in. I don't know, but with Longhorn two years away, what's in and what's out at that level of granularity is still very much in flux.
And then, the really critical questions would be:
(1) Does Microsoft put their strategic and marketing muscle behind it;
(2) What version of RSS? - or God forbid, a new one (RSS.NET!)
(3) How does Microsoft define an aggregator?