It’s the one from Feb. 21 (if anyone can point me to an online site that shows the current strip instead of two weeks ago, I’d be happy to hear about it):
The best I can come up with was some kind of play on “Atlanta Braves,” but what’s with “Big Chief Tablet”? I thought maybe this cold I have was inhibiting my understanding, but Mr. S didn’t get it either and he’s healthy.
Please save what little is left of my sanity.
The Big Chief tablet was for many years the most popular brand of paper writing tablet among school children and hopeful novelists in the United States and exemplified the lined writing tablet as a communications medium. The tablet featured a native American with full headdress on the cover. 
Hah! I was just about to post the exact same question before I spotted this thread, I don’t get it either.
Here’s a direct link to the cartoon, to avoid confusion:
http://est.rbma.com/content/Bizarro?date=20040221
I love Bizarro, and the humor is subtle sometimes, but it’s rare when I don’t get it.
OK, I’ll buy the “Big Chief Tablet” explanation, but I still don’t see how that ties in with “Atlanta.” Now I feel denser than ever . . . and I usually love Bizarro!
What ouryl failed to mention was the Big Chief tablet was used by 1st through 3rd grade, while they were learning the to write. The lines were very wide & the paper very cheap, like news print or less. You could get splinters from it. One used a pencil with a 1/2 inch diameter lead.
I don’t know if that helps. I’m completely sports illiterate.
It’s just an introduction of one stereotyped American Indian mascot to another. The “Big Chief” is meeting the “Atlanta Brave.” The joke (such as it is) is that neither of these characters has any represenative accuracy as Native Americans.
It’s sort of akin to introducing “Aunt Jemima” to “Uncle Ben.”
Or, a la Gary Larson, when the Elephant Man met the Buffalo gal?
OK, I guess I can see that. Duh. Now that I’ve completely ruined it by forcing you all to analyze it for me. Sheesh.
Well, it was kind of forced humor anyways. Usualy, he’s much more funny.