I was thinking about this thread the other night, and I've come to the conclusion that it's ridiculous to debate whether certain comics are theme-driven or character-driven. Terms like that are reserved for literature, which are stories with beginnings, middles, and ends (not necessarily in that order in some cases). Comics that do not have beginnings, middles, and ends - like Penny Arcade - cannot be labeled as theme-, story-, or character-driven. It's like trying to determine what kind of an apple an orange is.
Trevor's comics are great examples of these because they are stories. Penny Arcade, XKCD, et al, are just comics with a particular genre and subject (gamer humor, nerd humor, etc.).