'This release is dedicated to my grandfather, Walter Gordon Heffron, who introduced me to Unix 30 years ago, and who passed away yesterday.'

What is the use case for subrepos? http://mercurial.selenic.com/wiki/subrepos

It it analogous to svn:externals ?

The Mercurial subrepos are similar to and inspired by git's submodules (but, presumably, less user hostile). You can use this to group your app with the libraries it depends on, for instance.