PartialClone feature -- how hard would it be to do?

[Chris Quenelle]

There is a wiki page here:
  http://www.selenic.com/mercurial/wiki/index.cgi/PartialClone
and an issue record here:
  http://www.selenic.com/mercurial/bts/issue105

that describe a proposed feature "Partial Clone" which would allow you
to create a clone with only some subtrees of the your sources.  Not
just in your working directory, but also in the history DB itself.

I've always assumed this would difficult because of the way the revision
checksums are created and used globally.  But I can't find a more in-depth 
discussion of how hard this would be.   Was this addressed in a previous
email someplace?  Maybe I could copy/paste that discussion onto the wiki 
page?

How hard would this be to implement?

--chris