I did some number crunching on the historic database, trying to get a better idea of what players have had careers like Soriano has had so far, and how they have progressed after age 30 (his seasonal age in 2006). Here are some thresholds I used to screen the data:
batting seasons from 1960-2005.
batters who had 3000+ AB through age 30 (Soriano has 3900).
batters who played at age 31+ (this mainly got rid of active players who aren't 31 yet).
I then compiled stats for all these guys through age 30, and for ages 31+. Then I went about screening the "through age 30 stats" to get guys who are at least somewhat like Soriano:
players with .450+ slugging through age 30 (Soriano is .510).
players with a K:W rate of 3+ (Soriano is almost at 4:1 for his career).
players who stole a base after 2% of their plate appearances (or more. Soriano is at 5%).
In a strange twist of fate, these screens found 3 players, and all three are ex-Cubs. Go ahead and try to guess... I'll put the rest of my research in the Comments below....