Dave Roberts is not a free agent, so he will remain on the Red Sox unless a trade takes place. He won't start, with Ramirez, Damon, and Nixon from left to right. He's also on the Red Sox, who are in the same catagory as the A's: the Bill James persuasion. That being said, Roberts probably isn't going to rack up the SBs.
If Lofton is traded, he could steal 20+.
BJ Upton on the Rays is quick, and he'll probably be in the majors on Opening Day.
Soriano has speed, but the Rangers never sent him, so I don't know what to expect, trade or no.
Macier Izturis stole 4 bases without getting caught in 32 games, if he starts, that translates to a little over 20.
But what I've found is that, instead of getting 2 big SB guys, get some who fly under the radar in that catagory. Derrek Lee, Brad Wilkerson, Jason Kendall, Michael Young, Bret Boone, and Derek Jeter are all valuable in this light. Having multiple guys like this accumulates steals without dependence on a single player for them.