The -vt option of emerge is very useful for finding strange dependencies when doing world updates. E.g. when the question is “Which ebuild is causing X11 to be installed on my headless server all of a sudden?”. Just add -vt to emerge -puD world:
emerge -puDvt world
..and take a look at the root of the relevant tree. In this case, the culprit was “fte”:
[nomerge ] app-editors/fte-20050108-r3 -X +gpm +slang [ebuild N ] virtual/x11-6.8 0 kB [ebuild N ] x11-base/xorg-x11-6.8.2-r6 -3dfx -3dnow (etc.) [ebuild U ] x11-apps/ttmkfdir-3.0.9-r3 [3.0.9-r2] 19 kB