In pre-EAPI-2, src_unpack phase was the most logical phase to be provided by 
SCM eclasses, thus classes has been set up to export ${ECLASS}_src_unpack.

This phase in most (if not all) SCM eclasses provided:
- unpack functionality - fetch and store in ${DISTDIR}
- bootstrap functionality - either patching or bootstrapping just unpacked 
sourced with command

In EAPI-2, bootstrap functionality belongs to prepare phase, thus it's been 
moved there - SCM eclasses in EAPI-2 codepath has been set up to provide 
src_prepare.

And this is the problem (some people may be even unaware of it).
In pre EAPI-2 it was sufficient to do the following in live ebuilds:

inherit ${some_eclass} ${scm_eclass}

${scm_eclass} inherited as last one, would just shadow src_unpack, providing 
what we want. In EAPI-2 however, it as well shadows src_prepare which is in 
*most* cases unwelcome, especially if one uses autopatcher (base.eclass, so 
cmake-utils.eclass, kde4-*.eclass, and probably some other).

Because SCM bootstrap is either not used at all, or used very rarely, there's 
suggestion to:
- either drop it
- or (preferably) to make SCM eclasses export src_prepare only on specific 
request
- or to make it easier - to not export it at all - thus making it required for 
developer to intentionally invoke ${ECLASS}_src_prepare if bootstrapping is 
required.

-- 
regards
MM