Viewliner,
To my knowledge, my line the #7, is the only line that still has redbirds on it. At present I would have to say that there are at least 100 to 150 cars still in service. That number may even be higher, but I'm sure that it's at least 100. I've seen as many as ten trains at one time using redbirds, so with 11 car trains, that puts the number over 100.
These cars will eventually swim with the fishes too, but I think that it's still a few months off before they are retired. At present probably about 2/3 of the #7 cars are the newer R62 cars. The remainder are the R32/R36 Redbirds.
Amfleet,
As GS mentioned, the cars are completly stripped of anything so there is no environmental damage. Once in the water they allow coral reefs to form. Cars that were dropped off of the Carolina's a few years ago are already paying dividends. The local fisherman are loving it, as the fish love the reefs. They say that the areas where the cars were dropped have seen at least a 50% increase in the number of fish caught in those areas.