http://mlb.mlb.com/NASApp/mlb/mlb/offic ... ations.jsp
Determining rookie status:
A player shall be considered a rookie unless, during a previous season or seasons, he has (a) exceeded 130 at-bats or 50 innings pitched in the Major Leagues; or (b) accumulated more than 45 days on the active roster of a Major League club or clubs <b>during the period of 25-player limit</b> (excluding time in the military service and time on the disabled list).
I suppose the "during the period of 25-player limit" is the confusing part. In September, the roster limit expands to 40 players, but perhaps this clause is only related to (b) and not (a).
Case study: Matt Kemp. He's got 122 at bats this season. He'll probably only reach 130 in September when roster limits expand to 40. BUT, if that clause only applies to (b) and not (a), then he'll lose rookie eligibility. If it applies to both (a) and (b), he'll keep his rookie status. I wonder who the arbiter is on this score. Any historical examples to settle the matter?