The cool thing about a series if it does get canceled is that once it does there ain't jack anyone can do about it.
The only thing that came close to uncancellation was Shaman King but even then that was only so it could have a coherent ending and even then that was taken in by reader votes and volume sales or whatever.
Not that Bleach is going to get canceled anytime soon. If it stays in the bottom 5 consistently and it's volume sales drop a bit it could get canceled in 1-2 years. But even then no gurantees. After all Reborn (correct me if I'm wrong) stays lower in the magazine and still runs right? Granted it's not bottom 5 usually last I checked.