In-game advertisements in sports titles aren't that conspicuous because, hell, real-life playing fields are littered with them. But screens in MLB 2K9 show publishers are comfortable with alterations of reality, if they turn a buck.
PastaPadre, a dedicated sports video game blogger, took a look at 2K9's screens of every stadium and noticed several inconsistencies. Wrigley Field has an extra billboard beside its iconic scoreboard to host a State Farm ad, presumably because its ivied brick wall prevents the obligatory plug. The State Farm ad is out of play, so it won't really affect the game experience. But in other cases, walls and outfield dimensions have been modified slightly to accommodate advertisements.
Now, it's not like seamheads are out there with protractors and tape measures, ranting about esoteric inaccuracies. Marginally extra height on a fence would affect what, 1 out of 1000 batted balls? I dunno. Baseball geeks are the kings of the sample size, and you probably need a big one before this gets to be a performance issue.
But it does exemplify advertising creep, and the extent publishers will go to modify the in-game feel to either make a buck or honour a one-size-fits-all advertising contract. (I'm certain MLB, MLBPA and State Farm were all guaranteed outfield scoreboard placement in 2K9. Hence the ridiculous ads on the apartment roof across Waveland Ave. at Wrigley.) And if this is the precedent, then what is next? It's like Ralph Nader once said. They'd sell advertisements on your eyeballs if they could.
Update: Here's a comparison of Nationals Stadium in Washington. Note how distorted the left field wall is to accommodate the State Farm advertisement. That definitely affects realism of gameplay.
MLB 2K9 Going Overboard With Stadium Advertising [Pasta Padre, thanks Josh]