Before the season started, the general belief was that the Padres would be among the top sellers this season, with Adrian Gonzalez and Heath Bell representing the prime pieces of trade bait. The team's surprisingly good start has changed that belief, and ESPN's Buster Olney hears that they will have some flexibility to add talent at the trade deadline if they continue to play well (Twitter links). It's the ripple down effect from last summer's Jake Peavy deal, since the team wouldn't be able to add payroll this year had they not made that trade last year.
San Diego started the day two games up in the NL West thanks to a 29-20 record, and their +41 run differential suggests it's no fluke. Their success stems largely from pitching and defense; they boast MLB's best team ERA at 2.98 and the second best team UZR at +19.2. Offensively, the Padres have hit just .242/.319/.359 as a team, though their lack of power is somewhat mitigated by 54 stolen bases, just one behind the Rays for the league lead.
Padres' left fielders are hitting just .186/.286/.299 on the year while their shortstops have chipped in just .219/.278/.313 worth of offense, so those are two obvious areas that could be upgraded. Corner outfielders are always in ample supply at mid-season, but quality shortstops are typically harder to come by.