Upton is not, by most accounts (notably the people who matter in Tampa), defensively-capable of handling SS in the big leagues right now. He's quick and has good reaction time, and makes the big plays, but his form and mental lapses keep him from being even remotely consistant on the easy plays... OK at 3B, but a big no-no at SS. Every indication suggests he will play 3B the majority of the time in Tampa.
Also to note, not only was Zobrist called up to share time with Tomas Perez at SS... but Joel Guzman, the absolute stud Tampa got for Lugo, projects as either a SS or an OF... and Tampa has less room in the OF than the IF. Everything I've heard suggests the Rays have an eye on Upton/Guzman on the left side of the IF for a couple years, and they'll trust Guzman at SS over Upton at least this year and probably next.
Worth keeping an eye on though, as Guzman to 3B is probably more logical in every area with the exception of defense. But for the time being, expect Upton to qualify at 3B well before he's shifted to SS full-time.