There are for example SharePoint questions on SO, SF and SharePointOverflow.
It would be nice to be able to see / search for answers across all sites.
Stackexchange sites like SharePointOverflow and SO, SF are run by totally different people. They use similar software, as StackExchange is a version of the SO software that is further developped by FogCreek (Joel's company).
It would be rather hard to make a search to span all these sites, apart from existing solutions like google. Obviously SharePointOverflow will not be indexed as fast by google as SO and SF.
Apart from this, searchability across sites has been rejected several times even for trilogy sites only:
This question is a bit out of date, but under the new SE site model, there is now a centralized hub of all sites, stackexchange.com. On that site, you can:
This does not work with the remaining SE 1.0 sites, as they are being phased out.