I’m running Gentoo; which version of Eclipse did you grab, motif or GTK?
If you are using motif, you need XFree installed and openmotif
If you are using gtk, you need XFree installed and GTK+
Also when you mentioned typing “./eclipse/eclipse” please describe where Eclipse is installed and as what user you installed it under. Did you emerge it or just download/unzip it? Sometimes programs care where/how they are launched, so try cd’ing into the ‘eclipse’ directory and then launching it with ‘./eclipse’ that should work fine.