When someone says a comment that is totally irrational, irrelevent, a dumbass comment, makes no sense, shows signs of deleriessness, or when someone you don't know jumps into your conversation... 'about that' is perfect in those situations
-Check out the new psp i just got yesterday
-Damn that thing is tight, i'm thinkin bout getting one now
-(Random kid) Dude is that the new Psp!!
- "About that......."