It would be extremely useful if emerge -p (and -ep) took account of all the ebuilds specified on the command line when building its dependency tree.
For example, currently "emerge -p gentoo-sources nvidia-kernel" will pull in, amongst other things, everything required for gentoo-sources, then a default kernel (generally different, depending on the virtuals file), then nvidia-kernel itself.
A second example: on a system with no apache or mod_php installed, "emerge -p =apache-1.3.29-r1 mod_php" will pull in everything for apache1, then apache2, then mod_php itself. Obviously this doesn't happen if apache1 is installed before doing "emerge -p mod_php".
*** This bug has been marked as a duplicate of 1343 ***