CB Al Harris had a great 2008 season, if for no other reason because he came back from a serious mid-season spleen injury. He was better on the field too, he wasn't beaten as badly as he was during the 2007 season. He had some awful games in 2007, capped by getting schooled by WR Plaxico Burress in the NFC Championship game. 2008 was a much better season for the secondary as a whole.
While S Nick Collins and CB Charles Woodson deserved their 2008 Pro Bowl invites, I'm not sure how Harris got invited to the 2008 Pro Bowl as an injury replacement. As I said above, he had a very good season, but he had no INTs and wasn't the playmaker either Collins or Woodson was. I haven't watched all the starting CBs in the NFC last season, but I have to imagine there were others that outplayed Harris. Carolina's secondary was outstanding last season, maybe CB Chris Gamble deserves the invite instead? I would have even selected CB Tramon Williams over Harris. Maybe it doesn't really matter since no one cares about the Pro Bowl anyway.