emerge -f kde-cvs (for instance)
does not use CVS to download the sources.
Downloading occurs at the 'compile' stage.
Steps to Reproduce:
This is a fix I'd like to see as well. I live in a fairly rural area, and
my only options are dialup, satelite, and nothing at all. Prefetching source
is the only way I can maintain my sanity and still use Gentoo (it's helpful
to start up a 'emerge -f whatever' session at night.)
If you're working with the KDE cvs builds, you could always give cvsup a
go, follow the official KDE instructions on grabbing the source by cvs, put
the source in the proper place, and when you go to emerge the builds, set
ECVS_SERVER to 'offline', which should disable fetching. Note that I've
not tried this yet, so YMMV.
This isn't currently fixable due to a portage limitation which prevents overloading of the _fetch functions.