The Pats are still prepping O'Connell to take over if needed in a few weeks.
Cassel has a variety of issues, some of which may be his own and others which may be coaching staff inflicted. The biggest issue right now is he has no internal clock or pocket presence. On plays where he is about to get clobbered he will stand their oblivious. On plays where he has time and a solid pocket he will pull the ball down and run . . . usually into his linemen or taking a sack he didn't need to take.
Similarly, his reads many times are suspect. There have been a ton of plays where he had receivers wide open and didn't see them or opted to throw to players that were much bettered covered. He also has had several drives end by throwing 5 yards short of a first down on 3rd and long. All this has led to drives that have stalled prematurely, field goal attempts, and up until last week a shortage of points.
Against the Niners he did have the nice bomb to Moss, but the rest of the game was nothing to write home about. Without that one highlight reel play, Cassel only had 193 passing yards with 0 TD and 2 INT the rest of the game.
I've been saying for a month now that Cassel with stay as the starter as long as the team is winning and the offense starts to score over 20 points a game. If they are 4-4 at the mid point of the season (very possible IMO), they will probably stick in O'Connell.
For those that asked about Cassel's progress compared to Brady, I would say they are about even. Through 3 games and a releif appearance Cassel has totaled 707 yards, 3 TD, and 3 INT with the Pats 3-1. Back in the day, Brady had 664 yards, 2 TD and 0 INT with the Pats 2-2 in those games.