It's Arial, or Helvetica, which has been bolded. They're both nice, easy-to-read fonts, and from that angle, nearly indistinguiashable without up-close scrutiny.
If Southwest did their registration templates on a Mac, though, then it's more than likely Helvetica.
There's actually no regulation as to [i]whatp/i] font must be used for a registration, just that it meets certain size requirements, and that it must be legible from a given distance.
[Edited 2013-04-09 02:28:28]
Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.