Here are the slogans on the team sites for '09. Some probably just haven't put theirs up yet, so I'll update in April, if I think of it. Compare to last year if you like. Not too many good ones this year. The Mariners are still the champs in my mind, in terms of creativity. Also note that some teams sites have official sponsors--the Sox had that Chrysler Mini logo on there, but it's not there now. I hope they're going sponsorless as opposed to being in between sponsors.

Angels: What A Year! (With the A being their logo.) (last year it was What a Game!) And the AL West Champs logo. Update, 3/19: Fan Loyal. Fan Faithful. Fan Strong.
Astros: No slogan, but "Celebrating 10 Seasons at Minute Maid" logo.
A's: 100% Baseball. (same as last year) Update, 3/30: 100% Athletics 100% Baseball. And they're using the yellow A prominently.
Blue Jays: No slogan. Update, 4/11: You Belong at the Game.
Braves: The Excitement of Turner Field. Update, 3/30: The Magic of Turner Field.
Brewers: No slogan. Update, 4/28: Now they have a temporary slogan: Vote Early. Vote Often. Vote Brewers. They also seem to have gotten rid of that wheat logo.
Cardinals: No slogan, but All-Star Game 2009 logo.
Cubs: No slogan, but 2008 NL Central Champs banner. Update, 4/18: Now showing '07 and '08 NL Central banners.
Diamondbacks: No slogan.
Dodgers: Dodgertown California. Update, 3/19: The little "California" has been changed. Now it's "Dodgertown USA". Further update, 4/11: "This is My Town" with "Dodgertown USA" next to it.
Giants: Your SF Giants. (with SF being the hat logo) (same as beginning of last year)
Indians: Are you IN the Tribe?
Mariners (always the most creative sloganeer): Mariners Baseball. A New Day. A New Way. (Also Safeco 10th anniversary logo)
Marlins: It's Where You Wanna Be!
Mets: No slogan.
Nationals: No slogan. Update, 4/11: "Welcome Home" (the W is the logo) Update, 4/29: Oh no. The Nats have become America's 100th company to use the "get your _____ on" slogan. Way to stay hip and current and original! It's "Get Your Red On" with a "NatsTown" slogan next to it. (Ooh, also orginal...)
Orioles: This is Birdland. (same as last year)
Padres: No slogan, but 40th anniversary logo.
Phillies: No slogan, but 2008 World Champions banner.
Pirates: Pride. Passion. Pittsburgh Pirates.
Rangers: Built for Fun.
Rays: No slogan, but AL Champs logo. (site presented by ST. Petersburg Times)
Red Sox: No slogan. (Site had been presented by Mini, but that logo came down in the last few days, now no sponsor--woohoo!)
Reds: No slogan.
Rockies: No slogan. (site presented by Toyota)
Royals: No slogan, but it does have a "Be Part of Royals History" banner, referring to Opening Day. Also, 40th anniversary logo. Update, 4/11: "Welcome to the Next 40 Years of Kansas City Baseball".
Tigers: Always a Tiger.
Twins: This is Twins Territory. And Metrodome patch.
White Sox: No slogan, but AL Central Champs banner/logo. Update, 4/11: "There Are Traditions. And There Are White Sox Traditions." Okay....
Yankees: No slogan, but new stadium logo. (Site presented by AT&T)