I really doubt both Daimler and BMW want to have their fingers burnt again into mega mergers that turn out to be absolute handfuls of unmanageable behemoths. Remember MG- Land Rover- Rover, the English patient? Remember Chrysler- Dodge-Jeep, anyone?
In the current situation they badly need to get their costs down as, remember, they build their cars mostly in Germany (high labour costs). I don't think there's a lot to worry about brand-character being dilluted at all. They appeal to different audience and could both benefit from each other : Mercedes from BMW's engines and BMW could find economies of scales on buying components. What's the problem using the same brake pads, radiators of air con units? That wouldn't dilute anything at all. Having wheels made in bigger numbers would drop unitary price for both of them, for example. Buying leather, rubber seals or wipers... would nicely add up. A and B class are old and never brought expected returns (crappy overpriced commodities for me) while the Mini is a huge success.
Both are badly suffering at the moment while Audi is faring less badly. Uniting their forces could steer both groups out of the downturn less injured than if they remain on their own.