I have seen interviews with several top BMW execs (both Motorcycle and Aotomobile divisions) which indicate that they are, in fact, considering it. They indicated that if they were to do so, it would not be primarily for marketing motorcycles, but was for marketing cars. BMW car owners, especially in Europe, are very much performance oriented, and like to be on the cutting edge of technology, much different from the Bike customers who are much more traditional.
The other reason cited was to motivate and reward their engineering and technical staff. I believe that Honda used to do that (maybe still do) -- rotate promising engineers through the race departments as 1) a reward, and 2) part of training.
BMW usually does pretty well in races when the company decides to seriously try to win. They've done pretty well in the past with the Paris-Dakar Rally. They have been testing the racing waters lately with the Boxer Cup series. I think that BMW could very design a potent GP bike if they chose to put their corporate resources into it. I would not even try to guess what type of engine etc they would theoretically choose to use. Anyway, time will tell won't it?