I agree that there needs to be a way to discover API endpoints.  Especially with the [brave new world of SE 2.0][1] potentially leading to an ever increasing number of API-ified sites out there.

However, there isn't a good place to shove this functionality just yet.

Currently, we only have two places it *could* go:

 - api.[stackoverflow|superuser|serverfault|meta.stackoverflow].com
   - not "global," and accordingly shouldn't know about all the other sites in the network
 - api.stackapps.com
   - more "global," than the above, but also a site onto itself; it'd be a weird one-off to also have it serve up a network map

A similar problem exists with getting user associated accounts.

In short, once there is a place for this data to come from it'll be made available.  Most likely, this will exist "out of band" from the regular API.

  [1]: http://blog.stackexchange.com/post/518474918/stack-exchange-2-0