Here's the video:
First off, thank you, MLB Network, for having the field mic'd up properly. That was great. If you'd like to hear the uncensored, NFSW version, YouTube has it up for the time being. Anyway, it appears Balfour screamed at Martinez to stop looking at him. Martinez wasn't glaring or attempting to stare him down -- at least not excessively, I don't think -- so this looks to be a case where the always-tightly-wound Balfour let his rage get the better of him while Martinez didn't back down.
UPDATE: Both men adressed the confrontation after the game:
Video courtesy CBS Detroit:
It's also worth noting that Balfour is pretty well known to annoy opposing hitters with all his screaming on the mound and that the Tigers probably weren't too pleased to be on the brink of facing a 2-1 deficit in the series.
So, really, tempers flared in a spot that isn't overly shocking and no one was ejected. Thus, the importance of this spat is simply entertainment. And highly entertaining it was.