X-rays haven't confirmed yet, but docs have preliminarily called it a break. That's a tough blow for the Giants who are already shorter on power this year than last, having swapped out Michael Morse for Nori Aoki and Pablo Sandoval for Casey McGehee in the lineup.
Pence is also one of mlb's iron-men, playing in all 162, and starting all but one of those, and starting every postseason game. While Gregor Blanco is a more-than competent defender in Right Field (think the AMAZING catch late in the game to preserve Matt Cain's Perfecto), AT&T is universally recognized as having the toughest Right Field to defend in the bigs. It'll be tough in the early-ons of the season to fill that hole offensively and defensively.
The silver lining to the sucky news is that it IS early in Spring Training, and he shouldn't miss all that much of the regular season. Hopefully he'll be able to get back up to speed quickly.