Expect to see a lot of this.
Single game tickets went on sale on Friday, and as of this writing (~11 pm Sunday night) there are manyof tickets available for OPENING DAY. Poking around the ticketing site, you can buy singletons in in Field Box (OF), Terrace Box (IF), Upper deck box (IF, and the best seats in any ballpark), Terrace Box (OF), and assorted upper deck areas. But there are still plenty of tickets available in the bleachers, as well as terrace reserved spots in the grandstand. You can buy up to 6 in those sections (and one or two more price tiers). Most of these are well down the outfield lines, but still. Opening Day.
The other big series of the season, against the Red Sox, is more uniformly sold out (save for Bleachers). That's not too surprising considering the number of Boston fans that will likely stream into the park that weekend. Somehow I don't think the Nationals are as big of a draw.
I used to feel that Pat and Ron's guess the attendance game was a bit of a cheat, since you pretty much knew it every day. Pat and Zonk are going to have to hone their crowd-guessing/marketing-department-bullshitting skills to win free Cubs tickets for their listeners this year.