Gotta go to Ft. Worth every year for work, and Hard Eight is about five minutes from where we stay. I'll eat there every night. This location is in Roanoke though, so it might be a bit of a drive.
Not too many great BBQ joints right in Arlington. If you are stuck to something around the ballpark, check out Crossroads BBQ. It is right across the street from the Cowboys stadium, which is next to the ballpark.
Smokey's is is far east Fort Worth right before the Arlington line. It is just a few minutes from the ballpark. Their Q is very solid but not the quality of Hutchins, Pecan Lodge or Lockhart (all further east and north). Two mid-cities places of note that would be a bit closer than Dallas or other towns... North Main (only open Fri-Sun) in Euless and Meat U Anywhere in Grapevine.