I propose that the prlock.py script posted in the forums here:
be added to the gentoolkit package. prlock.py uses make.conf's RSYNC_EXCLUDEFROM setting to build an exclude list that results in you being able to run 'emerge sync' and just sync the packages that you have emerged. If you want to emerge new packages, you can just run prlock.py with the package names as the argument list. I find this very useful since I run Gentoo at work, where my proxy doesn't handle full emerge syncs very well. prlock.py would be very useful to those with low bandwidth, diskspace, or those who just despise wasted space.
Extremely bad idea, great way to make dependency graph bomb out.