Three reasons why Dale Jr will not be driving a black #3 even if he signs with Richard Childress Racing (RCR):
1) It is simply too much too soon for him. The black #3 is legendary, and has almost become mythical since the death of Dale Earnhardt in 2001. The expectation of anyone, regardless of name, to get behind the wheel of a black 3 car owned by Richard Childress will never be met. Dale Jr wants to win Championships and he would be setting himself up for failure in the black 3 car. The mental pressure of driving that car would be worse than driving the 43 car. Why? Richard Petty didn't die while driving that number, and it is Richard Petty putting you in the car with that number on it so you must have his blessing to use it.
In the end Dale Jr would be doing it for the fans, because it is something that the fans want, and not for himself. Dale Jr is his own man, and must continue to strive to succeed on his own terms and not to try to live up to the expectations of fans who want to relieve the glory days of the predatory Black #3 car hunting down its next victim on the track. No matter how you slice it, it will not be Dale Earnhardt Sr behind the wheel of that car so the fans need to get over it.
2) Richard Childress is still making a lot of money off of the #3 merchandise and the legacy of Dale Earnhardt and to put another driver in that car, again regardless of the name, would mean a loss of revenue. I also wouldn't be surprised that there is some sort of joint merchandising ventures with the trademarked 3 that Richard owns along with the trademarked Dale Earnhardt signature that is owned by Teresa Earnhardt. Having another driver in that 3 car would jeopardize that joint business venture.
3) Dale Jr has always said he wouldn't mind driving the 3 car near the end of his career as a tribute to his father after he has accomplished what he has set out to do (or at least tried). Although you could expect it earlier if he happens to win a Championship (or two) soon. I would expect RCR to bring out the 3 car in 2011 for a special tribute season and then put it away again, especially if Dale Jr won the Championship before then and is driving for RCR at the time.
If Dale Jr does sign with RCR he might get his old #8 back from DEI, which I highly doubt but Richard and Teresa do have a bit of a relationship together so you never know. I would expect him to end up driving with the #30, #33, or even the #32 (made up of the digits 3 and 2 which are the two numbers that Dale Sr had on the side of his car when he won his Championships).
Personally, I like 6.5 as the new number, it kind of goes with that humorous Bud commercial where Dale announces he is changing his number to 6.5 and then says, "Just kidding" after panic ensues in the Earnhardt nation over the number change. Does NASCAR allow decimals anyway?