Wiesman previously worked at EverQuest's Verant and is currently the driving force behind WildStar's UI engine. He reports that not only will players be able to change the look and layout of the UI from the get-go, but WildStar will support Lua to allow for addons and mods.
He assures potential players that creating such addons won't be a frustrating experience devoid of proper instruction and support: "If you can program at all, I promise you'll be able to make an addon for our game. Our commitment is to make sure the process is documented, clear, and accessible. Promise."
[Thanks to Bill for the tip!]