Advice? No offense (well, very little), but let's start here, shall we?
And why this is important even in hacker'ish fields:
Ok, now that that's out of the way...
Slackware is my personal distro of choice. You will learn more on Slackware faster than on any other distro, but you may develop gray hairs in the meantime. You will be forced to learn the ins and outs of .conf files and be forced to edit them in a text editor.
Debian is only half a step higher in the usability ladder. Debian can be run just fine from a CLI, but you will be introduced to a few more curses-based wizards and handbuild (i.e. nonstandard) utilities in the process. Apt simply rules as the greatest package manager in Linux.
Going a little more up the usability ladder, we have the mainstream distros of Fedora, Mandriva, and SuSE. These are fairly easy to use from a user's perspective, but this usability tends to come at a cost of sugarcoating. Wizards everywhere and you really don't know what is really going on unless you really dig for it.
Then there's the newcomer, Ubuntu. Ubuntu has all of the advantages that the mainstream distros posses with the additional benefits of being Debian-based and looking just awesome and polished. I would give Ubuntu to my grandmother, but I don't think that I would recommended it for a learning distro.