Ah, I see. I was going off memory that Arial existed in Open Office. After double-checking my sources (OpenOffice.org 3.2 running on Ubuntu 10.04.1) I see that I was incorrect in that. Nevermind, then.
Ubuntu doesn't ship it by default because of FOSS-wankery in the style of debian-legal — the Microsoft Core Fonts are perfectly free to redistribute in their original compressed form.