Have I seen every MLB team play live? The AL is covered, I don't even think I need to check my files on that one. So let's (let me) go over the NL teams I've seen:

Mets, plenty of times. Saw them play at least the Reds, Cards, and Cubs at Shea in like first through fourth grade, because I used the games as my "show and tell" in school the day after. Saw the Giants and Rockies at Fenway last year. Saw the Braves at Fenway in '00, and at Shea a few times. Saw the Astros at Fenway in '02. Saw the Dodgers at Fenway in '04. Saw the Phillies and Pirates at their own parks, and at Fenway. Must've seen the Brewers in their AL days.

What about the Marlins and D-backs? Don't think so. And I haven't seen the Nationals yet. Padres? Have I seen the Padres? Oh, right, Nomar's return game, June of '04, with the crazy rain delay. So, it's probably those three. I'll see the D-backs this season, so in a few months I'll be two teams away from having seen every team in baseball live. Prize for that: none.